First Amendment Right is only for American citizens, no? If you're a visitor to the US for example, you don't get the First Amendment protection against anything, you're a guest. Why doesn't the same principle apply to a foreign company? I don't see how banning tik tok affects your first amendment rights or first amendment rights of American companies - maybe you can explain?
The Constitution binds the activity of the government, individuals are irrelevant. Congress is forbidden from passing a law that violates the inalienable rights of humans, freedom of speech and association being one that is conveniently enumerated in the first amendment.
You will not find anywhere in the text that limits this to citizenship (with the sparse examples of the concept of citizenship coming up being things like eligibility for presidential office). The purpose of the Constitution is to spell out the abilities of the government, and one of the things it is expressly forbidden from doing is passing laws that curtail peoples' ability to communicate or associate.
The constitution applies to everyone within the borders of the country, not just citizens. Tourists still get due process, can say what they want, cannot be forced to house american soldiers in their hotel, etc.
No idea if this applies to companies, but foreign visitors do get protections.