They can - if they follow Chinese laws. Which Facebook is unwilling to do.
In that case I have no problem with the U.S. banning tiktok until China relaxes its laws.
Trying to legislate other countries? You are aware that sword cuts both ways, right?
The U.S. can ban Tiktok within its own borders (for whatever reason) without making any attempt to legislate the rules for Tiktok in any other country.
Why do you have such a hard on for China or tiktok? As it is, the sword only cuts one-way. A Chinese social media company gets unfiltered access to western markets and populace but China wont allow the same in return. That's not a fair deal; so I say ban tiktok until China decides western social media companies can operate inside China with the same level of freedom that tiktok operates outside China.
Other countries are already trying to control US companies. Thats already the situation.
So there is no need for you to bring it up, as a possibility, when it is already happening.