The US is a valued trading partner of both HK and China. One doesn't generally go about pissing off their major trading partners unless there's something in it for them.

There are really only two things Snowden can give HK or China:

- Further intelligence. This seems unlikely if his story is what he says it is.

- A giant PR coup domestically, putting the Chinese government, for once, on the right side of a human rights/democracy/transparency issue.

That second part depends largely on how much the general population can be made to care about the issue. The Chinese won't hang onto a highly wanted American criminal that the US federal government desperately wants tried, not unless they can extract something out of the deal.