Amazon is the major seller currently, but if such a seller won't pick up a publisher, then that publisher either goes under or elsewhere. Publishers not led by sheer principles of free speech would likely cave and no longer publish. If that's the case, someone else would need to be willing, knowing that Amazon, and other sellers who would follow suit, would not be an option. It's an intentional cornering of a market to squeeze out undesired (by the seller) publications. Amazon is big enough to coerce other smaller sellers to follow suit, or face the wrath of cancel culture themselves.
IOW, don't need the government to infringe on 1A when you can convenience major corporations, media, and the masses to do it for you.