Even suppose that the govverment is not to ban the exchange websites, people still have reasonable incentives to dump btc and withdraw cash from it.

Even from your point of view, no inflow of RMB means the ratio of RMB/BTC is going low, at the assumption of no withdraw of BTC. Then the price will go down……that's the reason people dumping BTC to avoid  more loses.

Maybe they are wrong. but this logic is still a good explanation.