It's a rebrand + censorship then? I'm not sure the distinction you're trying to make exactly, it's not like Douyin is a completely different thing from Tiktok, so I think "completely separate platforms" is also a stretch. The core technology and parent company/ownership is the same for both.
Its functionally completely separate. Douyin content for people unter 18 its like educational material, science experiments, and pro social content. Tiktok for under 18 in the US is like bikini girls dancing, robot voices reading reddit threads while videos of people playing video games play in the background, and lefty social justice content.
No, it's not about that. It's about controlling the flow of information, which is totalitarianism 101. Every totalitarian government tries to do the same; it's just that China has perfected the toolkit and has the resources to implement it.
Like what? Are Americans downloading Tiktok under duress of some sort? Every American competitor has algorithmic video feeds, so there plenty of market substitutes.
This isn't about whether or not algorithmic slop is unhealthy. It's quite obviously awful for the people it's exploiting. It's about who's controlling it. This is without a doubt the proper handling of this from a foreign policy standpoint. The company went out of their way to prove exactly how a foreign government could use the service as a tool to affect policy.
That aside, people do use shit like TikTok, Facebook, Google, and other social media and adtech bad actors' services because they don't fully understand what they're signing up for. To a certain extent, people do use them under duress -- completely checking out from them is going to have a pretty measurable impact on how people interact with you. You will be an afterthought to a lot of people in your life. People will think you not consuming those products is strange. Interacting with many institutions you have to deal with will be harder. Many (maybe even most) people can't handle that.
None of this makes a case for government intervention. From what I can tell, you can be ostracized in a US high school if your text messages are in the wrong colour. DOJ isn't dragging Apple into court for forcing teens to buy iPhones to fit in better.
The economy is designed to get people to sign up for things they don't fully understand. Payday loans, sports gambling, margin trading. None of these are illegal, but they should all have strong public education campaigns so citizens know the risks. Social media should be treated the same.
The second paragraph of my post is completely beside the point, I just wanted to point out that it's a bit silly making the "downloading under duress" point when people _do_ do that in a lot of ways.
The law is not about whether or not it's healthy for people. It's about whether or not it is an uncontrolled tool that an adversarial foreign government can use to sway public opinion in the US. No government with any urgent sense of self-preservation would let that fly.