It's been always like that as a 'leftover' from our old version of the game (1.3.6). There was time when poleaxes had higher average attack than dual axes, because of how under-powered barbarians were back then. And they really needed that boost to actually kill something. Things have changed with Genesis (and then Descent patch) and poleaxe's attack has been lowered to match the attack of dual axes. I realize that some of you may say that Barb's weapon is still a bit OP, but lowering it to how it is supposed to be if we went PWI attack pattern, wouldn't be good move at all.
I wouldn't say barbs weapons are op at all, at the end of the day thats all a barb has. They have 1 skill attack which could be considered OP but it comes at a price and is rarely used in pvp. As a barb, after using a few initial skills the majority of the damage simply has to come from normal attacks and against HA users its pretty useless as it is. You already said it would be a bad move and I agree, dropping barbs attack powers will render them useless in PvP, and in many respects, thats what EPW is situated around.