Although there is no "wrong" way to DM this (aside from breaking the rules) I'll suggest the guidance I try to use when I DM:
Don't punish players for coming up with good tactics or strategies.
This particular combination falls into a broader category of strategies involving using monsters' attacks against them. In this case, the Avenger puts up their powers, then intentionally provokes opportunity attacks, risking damage to themselves, in order to reciprocate further damage on their enemies.
Opportunity Attacks are a fair issue, but I think the simplest thing for both DM and players is to have monsters always take every available opportunity attack. That makes things fairly predictable, and being able to predict the outcome of one's actions is an important part of making good choices. Players don't want to make a choice they think is good, only to have the DM twist things around and trump that choice.
Remember that D&D is about the players coming up with ways to overcome challenges. If they find a particularly good or effective way to overcome one particular challenge, that's ok. That's good, even! That's what the game is all about!
However, there is another side to this:
The DM has considerable leeway in using both strategy and role-play to guide monster behavior.
While predictability is important to make good tactical choices, unpredictability makes for more a interesting role-play experience. Finding the right balance between these two is something all the players have to work towards.
You may try to reach a middle ground that favours the players, where monsters make poor tactical decisions (like taking every OA even if it's bad for them to do so) to enable players to execute superior strategies. I'd say this is the "normal" approach most people have to D&D.
You may go to one extreme where the monsters have purely role-played motivations, and behave in a tactically unpredictable but role-playing predictable manner. This has the downside of frustrating tactically-minded players who expect the monsters to behave in a more "game" way rather than a "story" way.
You may go to the other extreme where the monsters execute the best strategy possible, where the DM uses every tool at their disposal to foil player plans and make combat as challenging as possible. This is very risky because the DM has no real limits on what strategies they can implement. Because the DM is in charge both of the combat situation and the combat scenario, they can introduce monsters that will explicitly be able to defeat the players, then be played to defeat them. As a result, I'd strongly discourage DMs from being too strategic.