1. This is a way of getting memes to the masses that we (the US political establishment) can't fully control.
2. Meta and others are getting their asses kicked revenue-wise by Tik Tok. Like any business, they'd use anything they could to fight back. Turns out they can use China fear mongering, so they are.
3. (Added) Believe it or not, there's nationalistic pride here. There is a reluctance to admit that an app from "the other side" (China) is more appealing to the masses than _our_ social media apps. Surprising then that we don't ban Hondas and Toyotas, even though they're far superior to American cars (but at least they're made by Asians who are _allies_ I guess).
No matter that banning an app is completely against the principles we claim, such as freedom for individuals, competition in a free market, and freedom of information.