In this example: who cares? But the problem is how implicit everything is.
Imagine that a major US ally (like Israel) were attacked by a globally recognized terrorist organization. Imagine if, for some reason, a high percentage of people on TikTok ended up being opposed to the US government's support of their ally. Imagine if there were protests across college campuses. And counter protests.
Would we know whether TikTok was behind the scenes, sowing discord? This is the kind of thing - weakening our alliances - that china would love to do. If china can reduce our willingness to defend our allies (think the Philippines in the south china sea, or Taiwan which.... there's explicitly a project 2027 in China to be ready to invade Taiwan)
Do we want the Chinese government to have the ability to do this?