The easiest answer is: because ByteDance said so.
ByteDance has a internal board staffed by the CCP, and their CEO Zhang Yiming has even:
> promised that the firm would in the future “Further deepen cooperation with authoritative [official party] media, elevating distribution of authoritative media content, ensuring that authoritative [official party] media voices are broadcast to strength.” [0]
> If "brainwashing" on social media exists then all should be made illegal not only the a small subset.
The US, like almost every other country, regulates international commerce differently than domestic commerce. The US intentionally makes it harder to regulate domestic matters in order to protect the rights of the people the government is tasked with protecting. They even go so far as having separate agencies responsible for domestic matters vs foreign matters.
[0]: https://foreignpolicy.com/2019/01/16/bytedance-cant-outrun-b...