I believe I have seen pictures of Hong Kong protesters waving American flags, so would imagine many might accept the idea. The flag caught my attention especially after I was shown pictures of protestors from Portland, Oregon waving Soviet flags. That contrast is just mind-boggling to me, especially growing up hearing stories about families who were sent to Siberian labor camps by the Soviet government and never to be seen again.
Rightly or wrongly, the US is still seen as a bastion of freedom and democracy by many around the world, and that's what the protesters are fighting for. More specifically, they are asking for support from western peoples and governments for a struggle that they believe free people can identify with. Seen after last week's "Hong Kong Way" event: https://photos.app.goo.gl/ARht6bNnEGTBFmnw9
Also, there's been a few right-wing/alt-right USians who've tried to hijack the protests for their own agenda, thinking they're among anti-commie friends. They don't represent anything mainstream, and they're not particularly welcome...