Do you disagree with my logic, or just the conclusions you assume I'm drawing from it?
I disagree on both.
You're framing player motivation as something inherently suspect. That because players express themselves through a single character, they have an incentive to push boundaries or act in bad faith for personal gain. But that’s an assumption, not a logical necessity. This is a cooperative hobby, not a competitive one.
It’s absolutely possible, and in my experience, common, for players to invest in their characters because they care about the story, not because they’re angling for power or advantage. Treating self-expression, like this, as inherently suspicious puts a strain on trust before anything has even happened. It promotes bad behavior from the GM as a form of inoculation against the possibility of bad behavior from a player.
It all frames players as naturally inclined to "get away with things" for personal in-game advantage, but that runs counter to well-established psychological and sociological principles, particularly the human desire for belonging, acceptance, and cooperation in group settings.
So, yes, I think the logic is deeply flawed. We should maintain a presumption of grace as the default or we risk a spiral of negativity that is unproductive or even harmful.
Edit: This is all based on how I read your post. If I misread it, I could be wrong on your logic.