Comment by hrimfaxi
Because a corporation is a group of people, and a group of people don't lose their freedom of speech just because they joined a collective.
And corporations can stand for things. They can have missions and use funds to effect speech in support of causes that align with their beliefs.
Is a corporation really a group of people? Of course people are involved with the corporation, but the corporation doesn't represent its employees, shareholders, management or customers. It's a separate legal entity with complex relationships with its employees, management, shareholders and customers, but with its own rights and responsibilities.
There are organisation forms that are a lot closer to being just a group of people working together, like co-ops and firms maybe. I'm not entirely up to date on all options in English-speaking countries (which will vary of course, but the Dutch Maatschap is probably as close as you can get to a company that's just a group of people.