Theft of bags happens occasionally. But most people want to retrieve their own belongings and don't want to take other people's bags, which may be heavy and generally contain items which are not that valuable. Some airports require arriving passengers to present their luggage tags before they can walk off with bags they have collected. But at most airports there is no real way to prevent someone taking your bag unless you arrive at the belt first and stand right where the bags are sent out and pay close attention to everyone.
An interesting experience in SHA recently:
I (white) walked up to the baggage carousel without difficulty. A traveling companion (Chinese) was stopped by an official and required to show a claim check before being allowed to enter the roped-off area around the carousel. Neither of us was checked upon exiting the area, they were only seeing if the person checked baggage, not matching the bags to the ticket.