And that's cool.
You do seem to be anti-DM to DMs who do not share your "use anything from any source book in any campaign" POV. Some DMs like certain boundaries and do not want free for alls.
Naw, heck, I limit things all the time. I put on all sorts of restrictions in my campaign. The difference is, I'm pretty willing to drop any restriction given a decent enough reason. If my only argument against the player boils down to, "Well, I'm the DM, so my view wins" I view that as a failure on my part. If I cannot convince the player through a mature conversation that the reasons I don't have X in the game are valid, then, well, I'll let him or her have whatever they want.
See, the funny thing is KarinsDad, up until the past three years or so, I was pretty much 100% DM. So, it would be rather difficult for me to be anti-DM. OTOH, like many people here, I've been gaming for about thirty years, most of that as a DM, in many, many groups and numerous systems. Telling me that you know better than I do how to make sure that everyone at the table is having fun just because you happen to be wearing the big daddy pants this week is pretty insulting. I play in a group where, in actual fact, I'm one of the
junior members of the group. That thirty plus years of gaming doesn't make me the most experienced at table.
It would be ludicrous for me to try to play the "Well, I'm the DM, so, I'm right" card. Good grief, collectively, there's about a century of DMing experience on the PLAYER'S side of the table. Me trying to claim any superior knowledge, again, just because I'm the DM? Yeah, not going to happen.
To DMs who do allow players to create parts of the world, can you enlighten me as to how this all works in practice? It sounds like it works for you, but I'd honestly like to know how. What happens if a player contradicts something already established in play? What if two PC backgrounds are mutually exclusive? You have to say "no" at some point, surely? Or is your world's reality in a constant state of flux according to the last person who spoke?
I once had a player mention (out of game) a back-story for her character that began with "she is the daughter of a King and...". She hadn't mentioned this to me while creating the character, or at any point in play. I could have just said "your character's culture doesn't have a King, so... no". However, I let it drop; the game-world didn't change as a result of what she'd said, and the issue never came up in all the years we played, so we never had to butt heads about it.
I love it when my players are creative with their characters, but they should at the very least run it past me first. In the example above, if she had discussed it with me while creating the character, I would have explained that her character's culture doesn't have a King or Queen, but I'm sure we could create the same emotional hook for her character in some other way. I try my best to accommodate, but I've never seen how it could work with everyone creating content. In some other game, sure, but not in D&D as I understand it. I'm fascinated to hear how people have got this to work.
To be honest, it so rarely comes up. Now, since your player didn't know that her culture didn't have a king, I assume that this was never established in game. So, how would it hurt the game to go with her idea? She adds a king to your culture, and you roll with it. Fantastic. The contradiction only exists in your head, because no one at the table knows (again, I'm assuming here.) Now, it's up to the player to not contradict any established campaign material, since that's bad form, obviously.
I mean, in the Dark Sun campaign I'm playing, I worked with the DM to add a quest for a magic item to my character (actually, I wanted to start with the item, but, it was a rare and the DM ruled that I couldn't start with it). So, fine, I had this quest and I eventually completed the quest and found the item. I then, out of the blue, added the idea that the item was part of a set, with another item linked to it and finding this first item brings connections to this second item. I didn't ask the DM beforehand, I just announced it at the table. Now, the DM could have ruled that I was mistaken, that the rumours I talked about were false, but, the DM rolled with it and now that second quest is a major element of the campaign. All because I have the confidence and the trust of the DM to add in things like that and we work together to build the campaign.