What in the world does the US being in the South China Sea have to do with the topic at hand: China sending their secret police into Hong Kong to "police" the protest movement (ie. detain and disappear people with impunity).
This knee-jerk reaction to complain about America every time China's misdeeds are reported on is growing tiresome.
The super powers are clearly aiming for a larger confrontation at this point.
If you want context to the particular change, look at neighbouring Macau where apparently it has been law for more than 10 years:
https://www.scmp.com/news/hong-kong/politics/article/3085725...
Two Sessions, two cities: what can Hong Kong learn from Macau’s experience of national security legislation?
I'm not saying people shouldn't worry about both, but if you're wondering why people are saying "but America" I'd guess that is the main reason.