Unfortunately it's not that simple. The immediate question at hand in Citizens United was about a group of private citizens being prevented from releasing a movie criticizing a Presidential candidate during an election. A movie costs money to produce and requires a large number of people to coordinate, inevitably shading over into a corporate legal structure... just like, say, a union, or a political party, or a newspaper. So how are you going to decide which corporate legal structures are making "legitimate" political speech, whatever that means, and which aren't?