"they" aren't doing it since the FUD we read is in English written by people who aren't even Chinese or care about what is really happening in China. for example when you go on reddit or twitter those people spreading the FUD on these platforms aren't from China. or if you look at Coinbase for example those manipulating the market and dumping aren't Chinese.
this is why even if China some day banned bitcoin for real nothing about manipulation is going to change since manipulators aren't from China. they've just created something big and scary and have been using it for years with surprising success.
But anyway there is not much to do, as long as people keep using social media to find out what is happening in the market and believe everything they read then this FUD is going to keep its effectiveness.