Except historically that isn’t true, giving them a platform has consistently resulted in fascists unifying disparate groups, spreading their views via lies and manipulation, and being viewed as more legitimate just because they have been given a voice, even if just by default. They then leverage that platform for their own purposes, degrading actual free speech. If you give people who are actively working to erode free speech a platform, then they will take that platform and dismantle it piece by piece. So if you want it to remain an actual free speech platform, you have to disallow groups that seek to destroy it. The decision isn’t being made on your behalf, it’s being made on behalf of the platform itself, and the people who would be harmed by fascist movements gaining more momentum. It would be like building a little bug house made of wood, and then allowing termites to move in. Except at least termites serve a purpose in nature and aren’t acting maliciously, unlike nazis.