Actually, apparently that's not true. Numerous posters here are telling me that the ONLY vision that applies at the table is the DM's and no one else's. It doesn't matter what reason the DM has for banning something. His reasons need not make any sense whatsoever. He's the DM and he's GOD, in Darren Drader's words. He can work in mysterious ways and the players should toe the line or get out.
No, you pretty much have it right. But as I said earlier, I've never had a player quit one of my games because I shot down one of his character concepts. Before we play, I go to the players and ask them, hey, would you be interested in a game where the premise is X, we're using Y setting, with Z rules allowed? If the answer is yes, then I develop it into a campaign. If the answer is no, then since I would have no players, I drop it.
Once a premise has been agreed upon, then I start nailing down specifics, which includes a list of exclusions. Maybe the exclusions say core rules only, or it might be that all splat material has to be checked for balance. Maybe it's that certain races do or do not appear in the setting. If it's a unique setting, then not everything in the rules has a right to exist. There are some things that I would never allow to exist, like when a player comes to me and says that they want to play a gestalt character or a dragon. Just because the rules exist in Unearthed Arcana and Dragon Magic doesn't mean that I'm obligated to allow them. On the other hand, if I were running an FR campaign and someone wanted to play a tiefling, I'd tell them to go right ahead - in fact I have. If I were running my homebrew, which has existed since the late '80s and has a very limited number of racial choices, for a reason, I'd tell them no.
The name of this thread should really be called Player Entitlement. DMs have been creating homebrews for as long as D&D has existed, yet this notion that the DM isn't the final arbiter of his own world is something that has arisen recently. Call me old school, but the concept that the DM isn't in charge of the game he is running is utterly alien to me. As a DM, I reserve the right to tell players yes or no depending upon whatever conditions I have set for the game.
If a player came to me with the attitude that some people in this thread are exhibiting, I'd bounce them. Just because someone is a D&D player, that doesn't mean that I have to invite them into my game. In fact, I have more players to choose from than I have spots at the table, so it's far easier for me to just get rid of the problem player than to accomodate whatever world breaking idea they're trying to shoehorn into my setting. Fortunately, in the 24 years I've been gaming, I haven't once run into a player who had a problem with handing over control of the game to the DM. When it does happen, I'll be sure to point them to this thread so they can join in with the rest of you who are saying what a jerk I am.