WHAT? Why is something thats there on purpose; griefing? It makes NO sense. Why would they allow you to steal then ban you? Thats so dumb.
PS if you get away with stealing & the owner didn't catch you. Can they still somehow find out?
Most servers do not allow users to steal other users stuff. Because Notch hasn't engineered any means of locking chests, there isn't much one can do to prevent theft except for hiding one's stuff. Anti-stealing rules are enfroced by kicks and bans.
If the owner knows when the robbery occured (it was all there at 5 p.m, was gone at 10 p.m.) the admins can check the server logs during the time period when the theft occured. Active users during that time period can be scrutinized to determine who is the theif. They could also revert the server (unlikely for a simple theft) so that it would be as if the theft never occured. (reverting server=loading a back-up of the server's minecraft world)