Actually, they can...stuff like withdraw HK from its list of no-visa entry scheme, or launch some serious reviews here and there including possible money laundering haven, or trigger a small cyber attack to crash HKEx servers so shut down a whole afternoon of trading....
Yeah, like U.S. is in the business of retailiation... makes her public image of a 'world police man' turn into an image of 'school yard bully' if she slaps HK with retailitory actions such as that.
U.S. wouldn't dare touch HK.