Several people have mentioned that they are ok with restrictions as long as it is not arbitrary. So I'm wondering, as a player, why would this bother you?
If a DM had no other reason to ban half-orcs other than because he doesn't like them, why does it matter? It doesn't seem to be any difference to me than if he said that his world doesn't have half-orcs. Either way, they wouldn't be available to players. Is it just the fact that you have no control and a "valid" explanation will calm your nerves?
I don't really restrict anything like that other than to ask players to choose from the 30+ list of races I do try to stick to (mostly because I'm familiar with them). Classes and everything else is pretty open-ended. I've been in games where we were restricted and I never felt I even needed an explanation. Even to the point where I had to play my least favorite type of character. I guess for me, I'm more curious where the DM is going with this. So I play the game to see what his world and adventures will be like with these restrictions (arbitrary or not). I may have preferred to play something else, but I can still have fun.