How?
It might expedite intel gathering and psychological warfare to an extent but let's not pretend China doesn't have agents inside of all of the "friendly" platforms already.
Because China can control what gets recommended to TikTok viewers and actively brainwash them. Even if it does not work on all people that's enough to be a security concern.
Yes. Human rights to free expression don’t end at the border, and are not contingent on citizenship.
If you only believe in human rights for passport holders, you don’t believe in human rights.
Furthermore, if you think your enemies having the right to publish whatever they like is a national security threat, you don’t believe in the nation or its founding principles as outlined in the US constitution.
State censorship is much more of a national security threat than foreign propaganda.
I think a lot of people don't consider the implications of foreign powers acting against their nation, not a corporation in that country but the actual government using it's vast resources to undermine other countries.