I think the main reason why they banned it - they cannot actually control it or make actual profit from it. But after they figure out how - china will probably legally enter the market.
I think you are right, China is banning crypto because it is very hard to control as this digital currency is decentralized. As i know China, they would not give freedom to it's citizen to enter the crypto as they are protecting them also to be victims of scams and frauds. If they will lift the ban this would give a boost to the crypto world as they are a major player once in this business.