I think the main thing, from a govt perspective, is that a popular social network not under the influence of the government is a real live-wire. The US wants to ban Tik Tok for the same reason that China bans Twitter, Facebook, Instagram, etc.
If it was actually about privacy or whatever, they would simply ban the privacy-violating practices, not the app itself.
If it was actually about privacy or whatever, they would simply ban the privacy-violating practices, not the app itself.