Beijing woud look incredibly suspicious wouldn't it? It would be hard to buy the "strong freedom of speech tradition" story.
Protection, balanced with plausible deniability.
Once you're protected, there's no more need for denying.
I mean deniability for the Chinese : Hong Kong is independent enough that they can claim powerlessness to their American partners, but they are the elephant in the room and can ensure that they keep actual control of the situation.
Why not? They're both in the sovereign territory of China.