Isn't it because the preamble begins with `We the people of the united states [..]` ?
> We the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defence, promote the general Welfare, and secure the Blessings of Liberty to ourselves and our Posterity, do ordain and establish this Constitution for the United States of America.
The preamble is who authored the constitution
Article 1 is all about how congress works
Article 2 about the president
Aritcle 3 about the courts
Article 4 about the states
Article 5 about the constitution
Article 6 about which laws are important
The Bill of rights further limits the power of government
Nothing specifically is about the people. The clause you state limits the powers of the state to take rights away from people.
The Constitution does not grant rights to people - because people (all people, globally) have all the rights by default. It does grant rights to government to do certain things, and it specifically limits the power of government to take away rights from people.
All the constitution has to say on firearm ownership is
> A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.
No mention of citizens.
The constitution does mention citizens
1) Limiting people from congress/senate/president unless they have been citizens for various lengths of time
2) The supreme court applies
3) States can't discriminate between people from one state or another
There was at one time the argument that the bill of rights codified some of the natural rights that were inherent in humans, to protect against government encroachment, as per The Rights of Man theory of natural rights https://www.ushistory.org/paine/rights/c2-04.htm but this is no longer the popular interpretation, generally theories that prevent power from being asserted will lose popularity, at least among the powerful.
That pretty clearly attaches "the people" as a qualifier only on the right to assemble and petition the government for redress, and not on the freedom of speech or the press. I'm not sure how you purport to parse that as "abridging the freedom of speech of the people", even if we do grant that "the people" refers only to citizens, rather than all subjects.
Yes, quite; there's room for reasonable disagreement about whether the first amendment protects the rights of assembly and petition only for citizens, for any subjects, or for anyone generally. But the protections for speech and the press (ie the ones actually relevant to Assange) have no such qualification. If they were prosecuting him for, say, standing in front of a government building yelling "Down with the NSA!", the fact that he's not a citizen would arguably be in any way relevant.
> Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
This doesn't say anything about whose freedom of speech, only that Congress may not abridge it. The existence of the freedom of speech is taken as a given, regardless of whose speech it is. The restriction is on Congress to not be able to abridge that freedom of speech, no matter who holds it.