I think it's more because the group gets filled immediately, than that they do not want you in. Usually people do not care about who joins loot groups (unless they really hate that one player) cuz all they do is boosting the loot, so anyone will work.
DragonProtector they got banned most probably because the guy that got attacked sent a screenshot containing his combat log proving that he got attacked. And since GMs are well, GMs, they won't take the time to play as a lawyer for every case like this, they ban on evidence of a crime.
Yes, I know what you say to be true too. Its just frustrating.