High Risk, High Reward Power
Brash Assault is one of those things where you can justify it with game-world logic, not using "player" logic. What the monsters / enemies of the PCs see is someone with a lowered guard. The more reckless (perhaps less wise) individuals would jump on someone as soon as soon as a weakness is exposed, not considering what position it puts them in.
Essentially, this power is trying to set up a gamble. The player is essentially saying "I bet we can do more damage to your side than you can to ours."
How to Use This as a DM
As implied above, low wisdom may mean more recklessness and less awareness of their surroundings, position in combat, and ramifications of their actions. A low wisdom character would, therefore, take that attack in an effort to kill something, ignoring the potential consequences.
Even some of the wiser enemies may even elect to get the free attack. Their logic would go as this: if the person who would attack me does less damage than what I do to the warlord, and I can take that less damage, I would totally attack them. The person attacking me may even miss! Also, if my basic melee attacks can trigger other powers, then I'm totally going to hit that warlord and make that attack hurt!
Also, the desperate creature would take that attack. Starving creatures, who just want a mouthful of warlord, could take this attack. Creatures who have a hive mentality would also take this attack, because their goal is to do as much damage as they can before they die. This can be especially true if the creature does something upon being bloodied or killed; I even know players that specifically want their characters to get bloodied to take advantage of things.
I'd look a short look at your monster's motivation for being in combat. This would give you a good idea as to why they're attacking and what their goals are. True, some wolves would generally not be so aggressive, but a starving one would. You may disagree with me, but I find that combat is more fun with opponents who don't always make the tactically best choice. Fun is the reason why we DM and play, right?
Enemies of the warlord should take advantage of this power because: 1) it is actually a good deal for them (free or low-risk damage), 2) they really don't know it's a bad deal for them (requires them to be unwise), 3) their goal isn't survival, but damage or something else (perhaps due to "bloodied" or other effects).