Yeah, this is the critical legal reasoning behind Citizens United, and it doesn't get enough respect from people who dislike the practical outcome of that decision. You do not surrender your rights when you join an organization. Anything that one of its employees could legally do is something that the corporation (which is, as the saying goes about government, just another term for things we agree to do together) should be able to.

It's complicated and difficult to draw the lines and impossible to do it perfectly. But if Elon Musk wants to blow an entire fortune for the lulz, that's kind of up to him. And we shouldn't stop him.