Citizens United determined that corporations have some of the same natural rights that people do. In particular, First Amendment right, which in turn means that corporations can do the same politicking (and political funding) that natural persons do.
A more charitable way to put this: you don't lose your First Amendment rights just because you joined with other people for commercial or other purposes.
A more charitable way to put this: you don't lose your First Amendment rights just because you joined with other people for commercial or other purposes.