And once again, we find that some people just can't grasp the difference between "socially acceptable behavior" and "what I can get away with."
Do you guys also stiff waiters for no good reason other than being cheap, deliberately give bad directions to strangers requesting your help, or refuse to give up your seat on the subway to a 90-year-old grandma with a walker? There are no rules against those things either. Obviously it's okay to do them because if it weren't, they'd be illegal. And since your like-minded friends do the same things, they can't be socially unacceptable!
"Oh, but it's just a game." Sure... and it's still possible to be rude, obnoxious, and a bad sport in a game. Just because it's a game doesn't mean there aren't real people playing it.
As to the original question: I don't see any reason why anyone would object to squads splitting up between countries. But there are cheesy things you can do that way that you can't do, or can't do as easily, without squadmates on another side - like playing "I'll let you shoot down my buffs now let me shoot down yours so we both pad our fighter scores". Don't know of any
rule against that, but it's cheesy and extremely unsportsmanlike. As is trying to sabotage other players on the team you're currently playing for because they don't choose to play like you think everyone ought to play. But I think we've been there already (about 600 times).