Is someone assuming that? It’s perfectly normal to approve of AWS banning Parler while reserving the right to disapprove of AWS banning some other service. I’m very confused by this mode of argument “if we ever let any company perform any action that’s a slippery slope because what if they perform other actions???”

Nothing is stopping them from changing their rules. But if they change their rules from something good to something bad, one simply changes one’s opinion of their rules.