It might help to understand why Warlock's Curse works the way it does.
Premise: D&D 4th edition is, first and foremost, a combat game, not a fictional-life simulator. All the combat mechanics are designed to facilitate combat, not replicate reality.
4th edition is very much inspired by game design advances made in video game RPGs, particularly MMORPGs, and have taken note of the "threat" or "aggro" mechanics in these games, whereby players can control which of their party members the monsters are attacking. DND4e captures this mechanic in part through Marks placed by defenders and soldiers. The players use marks and controller powers to manipulate the monsters and the flow of battle.
Monsters, meanwhile, have relatively fewer mechanics, especially to control ranged player character attacks. They were able to offer each other cover, but the single biggest mechanic at their disposal was proximity: if the soldier monsters were in front of the artillery monsters, the soldiers could do some degree of protecting them, by making it treacherous for melee characters to walk past and negate ranged strikers from using their striker mechanic on monsters in the back row.
This, at least, was the PHB1 premise that DND4e started with.
It's worth noting here that PHB1 came with 3 strikers: Rogue, Ranger, Warlock. The Rogue's striker mechanic required combat advantage, which was either had through stealth or flanking. Flanking in particular was difficult to line up on the back row enemies, so the designers expected Rogues to work with the defender to take out soldiers.
Rangers and Warlocks had long-ranged attacks, and both had a striker mechanic they could place on a target: Hunter's Quarry and Warlock's Curse. Both had the "nearest enemy you can see" restriction. In other words, neither ranged striker could effectively use their striker mechanic on the enemies in the rear of the formation.
This, therefore, was the intention of the wording of Warlock's Curse: it was worded this way in order to give monsters the ability to use their formation to protect weaker members from strikers.
Why is it based on sight?
I puzzled over this a bit after reading the original poster's answer to his own question, and realized how important "sight" was to the question. The examples given are based on modifying sight, rather than distance, and the question lies in the importance of the sight requirement of the power.
I suspect the reason for the requirement is to prevent the power from being effectively blocked by a monster placed near the Warlock but cannot be attacked: for example, if one enemy were 3 squares away just across a wall while another were 4 squares away in plain sight, the one that can't be attacked would be the "nearest" enemy. If the designers originally wrote the power with only a "nearest" requirement, they must have been presented with this quandary and sought a rule to effectively turn the power from "nearest enemy" into "nearest enemy you can actually attack."
My intuition is that this power was created early on in DND4e's development process, and at that point they did not yet have the separate concepts of "Line of Sight" versus "Line of Effect." They may have merely had a vague concept that you had to see a target to attack it, and so thus that wording was a simple solution to the aforementioned problem.
Once the development process added extra layers of complexity and more terms were developed, this power was not readdressed. This results in a number of problems, such as being unable to curse invisible creatures (despite knowing what square they occupy when they are not hidden and being able to attack them), being unable to curse anything in complete darkness or when blinded, and being forced to curse an enemy you cannot attack if you can see it, for example if you have a Clairvoyance effect or are separated from it by a transparent wall.
This makes me think that the power ought to be worded "the nearest enemy to which you have Line of Effect" rather than "that you can see" as this would ensure that the cursed target can also be attacked with Eldritch Blast.
Thus I conclude that the sight requirement was there to prevent the power from becoming unusable in certain circumstances, not to actually limit its use or provide a means for players to limit it through creative manipulation of their current field of view. Perhaps the intention of the power would have been "nearest viable enemy" but how would one possibly define "viable" within the context of DND4e rules?
Is that wording still relevant?
DND4e has changed considerably with the addition of new classes, mechanics, and overhauls from things like the Essentials line of books. Many concepts and ideas have changed dramatically in the game's evolution, and many elements that still exist in the game don't serve the same function or otherwise aren't particularly valuable or important.
I would argue that the sight/range restriction of Warlock's Curse is one of those bits of legacy dead weight. No striker that has come out since PHB1 has had the "nearest enemy" restriction. The Avenger can apply their Oath of Enmity to any target within 10 squares (though they retain the problematic sight requirement). The Sorcerer applies their Sorcerous Power bonus to all attacks directly (with no sight requirement). Essentials variants of Rangers and Warlocks eschew the Quarry and Curse for direct effects (eg: Hexblade adds damage directly like Sorcerers and their pact boons trigger when the player reduces an enemy to 0, not when a cursed enemy is reduced to 0).
Since the logic for the mechanic (of forcing strikers to deal full damage to front-row monsters or lose their bonus striker damage to attack rear-row monsters) has been abandoned for classes developed since PHB1, it follows that the logic need not unduly cripple PHB1 classes anymore. However, this is just another fragment of DND4e's shambled mess of rules and yet another example of why the whole system ought to have been overhauled into a 4.5 rule set.
RP-wise, what can be said about this?
The nearest creature you can see is not the same as the nearest creature you are currently looking at
The rule isn't built around what you "are seeing" but what you "can see." In English, can specifically refers to the ability to do something, not the current state of doing it. When you ask someone "can I go to the bathroom" you are literally asking if you have the physical capability of urinating (consider instead "may I go to the bathroom" which is asking for permission). Thus, when the power says "the nearest enemy you can see" it means "the nearest enemy you have the ability to see" rather than "what you see with your current self-imposed viewing restrictions."
Again, from a strictly by-the-book rules-as-written, it refers to all enemies to which your character can trace Line of Sight.
Tactically, this does mean that you can intentionally block line of sight with a wall, and you can intentionally change what monster is nearest through movement (the teleporting warlock is particularly fun to play with this restriction in mind).
How can the power restrictions be explained? Perhaps the curse is an eruption of raw power from the warlock, which semi-autonomously seeks out the nearest victim. Keep in mind that the warlock gains its power through a pact with a higher source, and often a malicious one. That power source may have its own interests to satiate, and causing harm indiscriminately may play into any pact. The tactical struggle to get a particular enemy to be the nearest enemy in sight may represent the internal struggle the warlock faces in wresting control from its pact source. You might want to curse that archer back there, but the source is hungry for blood from that ogre right in front of you.