Checking Knives on domestic flight in China.

My friend tried to take 3 knives(all closed and taped up) in a checked bag from Xining to Beijing but they were taken out and I had to pick them up. The postal service doesn't send Knives so I am know trying to figure how to send them back to America. I thought you could take knives in checked baggage though? Any thoughts on why they weren't allowed on?