It's because this isn't about privacy at all, it's about a popular social media platform being outside the control of domestic intelligence agencies. The US is unable to control the narrative on TikTok the way they do on American social media, which allowed pro-palestinian sentiment to spread there unhindered. It had a huge effect on the politics of the younger generation (IMO a positive one) by showing them news and first hand accounts they wouldn't have seen otherwise.

Edit: And yes, China is able to control the narrative on TikTok and that is a potential problem, but so far they've had a fairly hands-off approach to US TikTok aside from basic language censorship. I figure the way China sees it is that an unmoderated free-for-all will do more to sow divisions in the US than a carefully controlled (and therefore obvious) pro-China narrative ever could.