It's not that it is allowed, it is that GMs do not want to get involved with grey area disputes and it is easier to let the kick happen than take a side. It's not a case of being allowed to kick people based on differing playstyles (please don't have this mentality Bixillarla), it's that disputing it is out of the GMs hands because it is a personal matter that doesn't break the ToS.
What you can do, is if you are being cussed out for DPSing as a healer, you could dispute it on the grounds of harrassment. It's actually more difficult to dispute a heal only healer being kicked because they can legitimately be kicked for being AFK too much.