It’s the same old “personhood” fiction for doing things in groups that’s been around since Dartmouth College vs Woodward, which is over 200 years at this point.
Corporations are also entitled to other rights that can be exercised by groups, like not having their property searched without a warrant or seized without just compensation, or their contracts broken. They are presumed innocent in court unless demonstrated guilty.
The actual Citizens in question were getting together (uniting, if you will) to spend money and engage in overtly political speech in a tradition that goes back to Thomas Paine. They used a non-charitable not-for-profit corporation to make and to show a stupid movie about Hillary Clinton. They used a corporation because that’s what you’re supposed to use for things like this and the alternative is sending the money to one private person’s individual bank account and that’s got all sorts of problems. And the court found that was a valid way for the people who contributed to exercise their rights to free speech, because of course it is.