For those that aren't familiar. TikTok is pretty ubiquitous in the under 21 crowd while being practically unused by older people. It's sort of like when young people fled to Twitter and Instagram to get away from their parents on Facebook. It's basically the next Twitter for that age group.
TikTok (the community, not the company) is also quite critical of Trump. For instance, the buying up of rally tickets a few months ago so he would show up to an empty political event was organized on TikTok and we know that it pissed Trump off [1].
That level of age and political specificity in their audience makes it easy to target them for their ideas.
This type of laser focused ban on specific conpanies (rather than, say, disallowing data to be stored in China or banning Chinese owned companies from operating) reveals the nakedly political motivation behind this action. "China is bad" is just the excuse here. It's really about banning a popular meetingplace for a political demographic opposed to Trump.
[1] https://www.nytimes.com/2020/06/21/style/tiktok-trump-rally-...