If everyone who saw them kiss would have to sign something, that would be a shit load of paper :D!
I just think it is some kind of tacit agreement between people who are allowed backstage not to talk about what happens there. I didn’t take the “we’re not supposed to talk about that” as something with a real legal issue. I think it’s more that they know they’re doing something public and are not supposed to reveal something that happened in private. It’s just common sense. But it doesn’t make it less interesting, though…