IMO thiefs are one of the main reasons that UO is so awesome. If you care about what you have, you need to protect it. Thiefs and PK's are the sauce.
And thiefs do earn what they steal. They spend hours stealing crap and grieving for every decent item they get. think of how many times they make that run to the brit healer:D
If they really piss you off(quite often) buy some animals and block the healer, then see how long it takes them to come back....