I guess the police force would be worried about having a negative reputation, so robbing a small time person might not be worth the risk if it meant you wouldn't get the contract for the bigger fish. But at some point you could always just cash out by robbing your client.
I would also guess that the people you hire to protect your stuff would probably also accept payment to take other people's stuff so you would have a similar set of problems to medieval kings and their armies.
I would also guess that the people you hire to protect your stuff would probably also accept payment to take other people's stuff so you would have a similar set of problems to medieval kings and their armies.