If corporations are made up of people...and international corporations are made up of people from different countries...then how do international corporations have the right to free speech in America, considering that their employees aren't all American citizens?
The Bill of Rights describes rights not unique to just Americans. Those rights extend to any persons. However, the government can abridge those rights when a compelling interest is demonstrated. Foreign persons are prohibited from donating to campaigns directly or indirectly ( see 11 CFR 110.20 ) because of the government's compelling interest in preventing foreign influence in elections.
In summary, they have the same rights as any American except those abridged for specific reasons.
The constitution and its amendments serve as a document to define and limit the powers of the government. In the case of the first amendment it prohibits (among other things) the government from abridging a persons inalienable right to speak freely and express themselves through written or spoken word. The document takes for granted that these rights exist and attempts to prevent the government from limiting or taking away those rights. Those rights exists for everyone (as do all the other rights protected in the Constitution) regardless of what country they are a citizen of, where they were born, what color their skin is, what type of genitalia they currently possess, what their preference is for sexual partners, what religion(if any) they choose to practice, or what type of clothes they choose(or not) to wear.
In the Citizens United decision the court found that not that corporations are people but rather that they express the will of the company's leadership and thus act as a tool for those people who are the leaders of the company to express themselves. So what was being upheld was that the government could not abridge the freedom of the leadership. However the way the decision was worded leaves it open to interpretation that the corporation is a person. I can however see no objective manner that would lead a rational person to believe this was the intent. The wording is quite clear that for matters where speech or expression are concerned a corporation must be treated the same as a person since it is a tool of the people running the corporation. It should be treated no differently than a bullhorn in this regard.
I am not sure that I agree with that ruling myself, but the old rules were designed to allow some people to buy influence directly while others were forced to do it in the shadows. At least this allows it to all be done in the open.
The First Amendment to the Constitution of the United States provides (among other things) that "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..."
That applies not only to Americans, but to foreigners, and to entities (such as corporations) as well as natural persons.
So corporations are protected.