Yeah, personally, I kinda think it's a dick move too. But, then, I wasn't the one bringing it up. He didn't think he was being a dick. And, considering the amount of ink spilled over the last couple of years about the importance of D&D canon (cf. anything to do with Planescape or any 4e changes to monsters), I'd say there is a significant portion of gamers for whom this sort of thing matters.
Obviously, it didn't matter to me.

Heh, I was shocked to read the climate/terrain bit in the Monster Manual for 2e. I had been using manticores for years in all sorts of situations and never gave it a moments thought. But, the point here is, I was wrong. It's no different than trying to drop a tiger in Africa. I was 100% wrong here. Manticores, by the rules, don't live where I put that one. So, is the player really wrong for bringing this up? At the time, I was kinda peeved, since it ground the game to a screeching halt. But, looking back at it, it's not cut and dried.