In normal matchmaking games the AFK players would have been banned, but in griffball the object of the game is to score, not to raise your k/d by spawn killing. In this case it's a lose lose for everyone. The AFK players are violating the rules by abusing a glitch (getting credits without actually playing) and the active team is taking advantage of them doing this by spawn killing them to boost their k/d rather than actually playing the game themselves, which is also seen as cheating.
Either way it violates Microsofts terms and conditions, and obviously can result in a ban.
Being AFC for multiple games will get you credit banned because it's "cheating". Cheating here is defined as "abusing an unknown or known exploit to gain benefit." In this case, credits without effort. The first cases of AFK boosting were actually in firefight matchmaking (the 4 player one) where players would be AFK and rack up credits while the other players had to kill wave after wave on their own. Not only did this boost the AFK players credits, but also their commendations. Lately, people have been AFKing in other lobbies because even if you don't play, you still get credits. And yes, it is a bannable offense, and I'm sure the AFK players in question were banned as well.