Whoah. Our gaming experiences couldn't be more different.
We're moving away from discussing rules and into discussing gaming styles, and I don't mean to put my style over yours by what I say, merely to explain where I think we differ, but:
Of _course_ I've seen players have their PCs respond plausibly to in-game pressure, and done the same with PCs I've played! And of _course_ I wouldn't dictate what a PC does under pressure (short of mind-control magic effects).
In my game I currently have two PCs in the hands of a villain. The PCs have an extremely close personal bond, and each of them is a fanatic believer in their cause. My villain of course thinks of them as terrorists - the PCs OTOH say they're freedom fighters.

If my villain were to torture either of them for information, my guess is they'd die rather than talk. If on the other hand he were to realise just how close they are to each other and start torturing (even threatening?) one of them in front of the other, that might yield very different results.
I can't disclose at this point what he's actually planning to do or whether I expect any of this to come up in the session (we're playing tonight!), but there's no way I'd deprive the players of their freedom to decide what their PCs will do if faced with either situation - or with anything else involving RP interaction that either my villain or any of the PCs might come up with.