The more I think about it now, the more it seems like in CTF at least, you need to shoot typists. And when you type, you need to be able to shoot at any time without it being considered backstabbing. There just is no other way that won't imbalance the game.
Because typing is important for team communication, but it is not more important than eliminating a dangerous local threat. So both the typist and other player need to be able to go to blows immediately. The typist should just slam his 'enter' key mid sentence and start shooting.
An enemy in your base could steal your flag, an enemy in his own base will interfere with your flag carrier and an enemy in no man's land can be a threat to either.
The only exception is someone in the distant periphery, assuming he has no sniper weapon or the map isn't too open and assuming there is an out of the way spot on that map in the first place. And then he is imbalancing the game against his own team by taking himself so far out of action just to chat (unless his team has an extra player).
And finally CTF isn't scored based on frags anyway, so this isn't really a "free kill".