Here's the argument: https://www.fec.gov/legal-resources/court-cases/citizens-united-v-fec/

Free speech, including political speech, is a right clearly guaranteed by the first amendment. The document specifies no limitations on how much speech or what kind of speech that is. There is no "political speech", "no hate speech", etc. The federal government can't limit speech of people or of organizations of people simply because they have money and wish to spend it to speak on a political topic.

I don't think we want to be in a country where the government gives you an allocation of free speech to use and after that, you can't voice your opinion on a political topic through modern mass media anymore.