As the SCOTUS said itself:
“At the heart of the First Amendment lies the principle that each person should decide for himself or herself the ideas and beliefs deserving of expression, consideration, and adherence.” Turner Broadcasting System, Inc. v. FCC
2. A core principle of the constitution is that those rights apply to noncitizens as well as citizens. They are human rights, not citizen rights. It's significantly more ridiculous for corporations to have free speech than a government. They don't have less of a right to free speech because we don't like them.
You may not like TikTok, but this line of argumentation also would allow the government to prevent US citizens from e.g. reading Iranian newspapers or researching Chinese media. The polarizing nature of TikTok obscures the fact that US users are losing freedom of access to information in this decision.
there is a difference between 'US Citizen having access to information' and 'Hostile Foreign Govt having Direct Push Access for news to ~70% of a generation of Americans as they go from puberty to voting age'. US isn't discussing a Great Firewall, the information will still be available, it just won't be as easy for a generation to be indoctrinated (well, by a foreign govt instead of our own* at least. Theoretically our govt has our interests at heart more than 'hostile foreign power' - but that's a different fight we need to have)