Whether or not there are some rational voices on Parler is beside the point: Parler is not moderating the voices that support and plan actual violence. That means that any company that supports Parler, including Apple and AWS until recently, is potentially also complicit in that violence. Those companies have warned Parler that they need to moderate, and Parler has not acquiesced.

Apple and Amazon absolutely have the legal right to not do business with anyone, for any reason; and they have the ethical obligation to not do business with anyone openly proposing violence.