Because freedoms apply to US citizens. It works just fine.
In other words "freedom of speech" means that Congress doesn't have the power to pass laws that suppress speech itself, regardless of where it comes from. It's not that the first amendment applies to some people (and certainly not that it doesn't apply to others), but that it applies to Congress and limits what laws they can pass without qualification.
Firstly, they didn't expand or alter the Constitution.
I'm not sure where you got that from. Reading back and I don't see it anywhere.
Stated again, hopefully more clearly; by exercising restrictions on China they've reestablished that China is not "the people" described in the Constitution. So, it's not protected from congressional acts.
And second, they didn't need to do that anyway. Foreign policy is a power reserved for Congress and the President. No amendment ever restricted that power.
See Article 1.
Further, the protection of free speech in the first amendment does not even refer to "the people", but simply prohibits Congress from abridging the freedom of speech per se, without qualification or any exception for who is doing the speaking. Congress cannot pass laws that abridge the freedom of speech as a matter of actual effect, regardless of whether they are doing so under the auspices of exercising other powers.
Foreign policy powers don't come into it -- if the act restricts free speech, it is unconstitutional, regardless of what end Congress was attempting to pursue.
These cases go back and forth.
I would argue that it wasn't necessary to spell out that constitutional rights apply to citizens because the law is presumed to defer to common practice in cases where no law exists. That is, laws go back to long before the Constitution and those laws are referred to as common law.
In that sense; the king has lordship of his subjects, and no obligation to subjects of other kingdoms. He collected taxes, and provided protection from foreign threat and justice for his subjects.
Likewise, citizens of China don't pay taxes to the US, they have not that obligation because they are not subjects of the US government. Neither do they have the rights that are afforded a subject thereof.
Congress has no need to clarify that. It's presumed to be the case. That they mention it in certain amendments is incidental, despite the creative interpretations I've read, mostly from immigration proponents. (Not that I'm opposed, I'm an immigrant.)
No doubt, lots of people prefer to interpret this differently, especially when it's a right they need for a specific reason. That's exactly what China is doing here.
They are arguing about why the law should be interpreted in their favor this time. And next time they will argue the other way, as they do against their subjects.
It's up to the judge what it means in each case, but as a generally accepted interpretation, applying US law and protections to a foreign nation without some kind of formal alliance specifying more directly what is provided, is an egregious stretch of the imagination. It's ridicule, at best.
Freedom of speech is my right to be able to bitch about Biden or Trump without having to worry that I'll die in prison.
Freedom of speech is not a right of foreign entities sending me memes about them to manipulate who I will vote for in the election.
There's a law where foreign entities can't own TV or Radio in US. This might be controversial, but IMO it should be updated to also include social media as this is source where people get news from.