had to look this up for a moment, the mechanic has been around forever starting with D&D 3rd edition, Neverwinter Nights 1 and so on.
Actually, a mechanic for discouraging disengaging from melee has been around since at least AD&D 2e (and in some ways the 2e version of the mechanic is much harsher than just an Attack of Opportunity), which the IE games are based on. The rules for that were just never implemented in the IE games, for whatever reason.
I should be cautious about what I say about this, since the guy who kicks up the most fuss about engagement on the Obsidian boards is now a junior on GAF (hey Sensuki, I'm talking about you), and it would be nice if the topic didn't come to dominate this thread, but basically, there are people who play the IE games in a style that involves heavy microing of character positioning and movement. The engagement mechanics essentially obsolete this style of play, since you can no longer safely move characters around willy-nilly. So, those people are upset about that, because their chosen playstyle in the IE games won't work in PoE. Which I suppose is fair enough.
It is a specific case of the issue petghost alluded to, where you want to make a game better for the vast majority of players but as a result you end up pissing off some superfans. For every detail in an IE game you think is a bug, or a mistake, or a design oversight, or simply a poor decision, I promise you, there is someone deeply in love with it, and if you change or fix it they will come for you.