Constitutional right to free speech does not equal a right to a platform for that speech.

Exactly. That’s why communists don’t have a voice anywhere. It’s no one’s fault that rich and/or politically powerful people more easily align with authoritarianism and fascism and just so happen to own those platforms. It does mean that groups and individuals advocating for under-represented perspective languish in obscurity. And there’s no reason why those voices should be represented on any mainstream platform. They have a constitutional right to scream at the top of their fossil-fuel charred lungs about climate change on the corner of the street like any homeless person. They don’t have the right to do it on Twitter or Facebook.