Look if they have proven that the app of Tik Tok is spying on their users I think they have the right to ban it because first of all it is violating the privacy of its citizens and it is one of their first rights being a citizen, if the government doesn't act properly then they are basically not interested in protecting their rights. This is really not about the protection of their economy or the internet being regulated but the protection of their rights. And also I'm still wondering why these app stores like Apple and Google are still keeping Tik Tok available for download even if they have proven it spying to their users.
All apps are monitoring you in some way or another the issue at hand is that the relationship between the US and China is tense right now and Trump does not want China to be monitoring their citizens just as they do their own already on China.
Obviously this is not being proposed to actually protect US citizens and their privacy otherwise they will go after Google and any of the major social media apps available for download, this is just a way to try to diminish the influence of China something that seem impossible to do at this point.