Usually this is the sequence of events in Lusternia:
- Person A does something very bad to Person B.
- Person B guild, city, friends and family jump against Person A. Also, some others jump in the fun just to kill something.

In case of thief, usually Person B entire entourage would take it on Person A guild/city/family until the items were back (as experience cannot be given back) thus causing unnecesary grief to everybody in B area of influence. And then B would find himself guildless and cityless unless he's a very valuable person worth keeping whatever the cost. As there're no PK rules, people actually can chase out the entire guild of somebody else with little or no repercussion (Avechna, curses) and cause quite a bit of pain. So anything that causes a guild to look bad (like not keeping a leash on their thieves) is to be avoided. And OOC people would feel most of the time against the thief.

Moondacers can steal from enemies, of course said enemies were already attacking the moondancers so that wouldn't change things so much.