It's a common practice to not share details of what led to a ban.
People are getting banned because:
a) they don't read the rules;
b) they think the rules are dumb, that they know better who should get the loot (i.e, them);
c) they don't think people will report them for "that silly useless item";
d) they don't care to inform themselves about their class and hit Need whenever it comes up, like Nidhogger said;
e) they find it worth it getting the ban for some item, but still try to come complain as if they were ignorant of what happened.
Out of tens of thousands of players we have, the tiny minority gets banned, most never do, especially for stealing loot.