Hong Kong is not China. Well, it is, but it isn't. They have their own government. They are a special administrative region of China, just like Macau.
Hong Kong is not China. But Mainland Chinese can travel to Hong Kong and indulge in Bitcoin trading without any restrictions. That is the entire point. There is nothing that the Chinese government can do here.
People from mainland China need a visa to get into HK.... not that it's a hard visa to get but it's not as simple as driving down a few miles to another town.