However, people can be unreasonable. So how do you decide as a GM that a player just metagamed and made a decision or if the character is simply behaving in an unreasonable way?
It hasn't come up at either of the tables I'm DMing, at least not seriously. There are some jokes about bad things happening wherever the PCs are--the parties ended up in the same city, three months apart, and while the first party found the city welcoming to adventurers, the second didn't. Since there are players playing in both campaigns, there was some pondering if they could somehow not be the ones the city had gotten mad at. No one really metagamed in practice, though.
Because for me, it sounds like people don’t want to incorporate player authored back story because they want to control the narrative of the game. And then one step further, they want to control how the characters behave.
I think the absolute refusal to consider using a player-written backstory might reflect that, or it might reflect an insistence on a play-style that at least doesn't reward backstories at all.
I welcome backstories, and they can come up at roughly any point in a campaign, but I'm at the point where I'm going to start asking for shorter backstories because I don't want to spend an hour trying to pull the hooks out, and because the PCs are supposed to be closer to the beginnings of their stories than the middles, let alone the ends. I'm also trying to keep my homebrew world consistent, so the more stuff there is in a character's backstory, the more likely it is to conflict with what I have or require some thinking to fit in. I'm not trying to control the narrative of the campaign: As
@Sadras said, I'm trying to keep the setting at least somewhat consistent/coherent.
EDIT: I don't believe I have ever tried to control how the characters have behaved as a GM. I have had villains and NPCs do so, however, with roughly little success.