Sometimes they do - for instance money used in stings and busts by law enforcement might have its serial numbers recorded. If you're subsequently searched and

Sometimes they do - for instance money used in stings and busts by law enforcement might have its serial numbers recorded. If you're subsequently searched and found to have money used in a drug bust, there are gonna be some tough questions and the money might get confiscated. If someone uses a fraudulent credit card to buy something from you online, you're not going to be able to keep that money when the fraud's reversed. If you (unknowingly) buy stolen property you can be forced to give it back, and then it's up to you to go after the guy who sold it to you. If you knowingly buy stolen property you're in trouble. Given that BTC serial numbers are in plain view as opposed to stolen goods or dollar bills where you need a warrant, it's a lot easier to demand the BTC back.