The Communications Act of 1934 limits foreign ownership of many communication technologies such as TV. TikTok has easily more influence than most TV channels so it does not seem strange to limit its foreign ownership. If the purchase of US steel by a Japanese company threatens national security, surely the ownership of TikTok is also one.
Sure, but just because its law doesnt mean its just. If you are just talking about "the law" you are talking about something very different than everyone else. Even if its the law, its obviously a violation of the intent behind free speech to limit speech only to those who the government can intimidate. If the only way to have free speech is to be within arms reach of the government's threats you arent really a bastion of free speech, you just practice speech within the bounds of what the government will allow. And as we have recently seen, that can change dramatically depending on who is paying.