Feeding data to the CCP, helping them identify people who can be easily swayed into espionage. Say someone gets into a position of power. "Hey, remember when you were 12 and said this on tik Tok? Now we need you to be out bitch or we're leaking this."
Look at the things that have gone viral on Tik Tok, it's like their algorithm prioritizes things that are toxic to make American youth shittier. Kia boys comes to mind.
There is also the fact that China bans all American social media out of fear that we'd use it to manipulate their people. If they aren't allowing our businesses to compete fairly, why should we allow theirs? Also, they probably are projecting that fear because they are doing exactly that with TikTok.
The app has more permissions than most apps and is highly invasive. They sent a push notification to all their users based on Geo location saying who their rep was and giving their phone number saying to call them to stop this bill. That alone seems like a major abuse of power. They are using the data they have to try to sway the American politicians already.