The Constitution does not impel corporations to provide a platform for hate speech. It is mostly about limits to government powers or rights of citizens vis-à-vis their government.
It certainly seems politically slanted in a single direction. Either way, I hope more people join the Fediverse and get away from these walled-gardens.
I’m a staunch free marketer and First Amendment advocate. I think this is a freedom of assembly issue. Amazon should not be forced to assemble with these customers.
If anything, this same group has fought for a more extreme version of this freedom through its don’t-sell-cakes-to-gays schtick.
(To be clear, I support e.g. the Westboro Baptist Church’s right to speech. They say awful things. But they have the right to. If they started organising murders or lootings of gay-owned businesses, on the other hand, I would not consider their actions protected.)