It's part of US' safety & security policy to require pax to remain in their ticketed cabin. So, this isn't a question about what the FA ought to do. And, it's not done because F pax are to be treated better.
If it occurs and it bothers you, raise it to the FA and remind her that it's part of her employer's stated policy. Then report it to FAA and they may or may not take enforcement action against US and the FA.
If it doesn't bother you that much, forget it and move on.
Your choice.