You can't "slippery slope" when the quoted email makes it pretty clear that Amazon has outlined specific and clear problems that Parler has refused to address. Unless your business is being used to post hate messages, including explicit threats to kill people, your concern is unwarranted.
If it is, well, then you should probably reasonably actually be concerned about AWS shutting you down.
If it is, well, then you should probably reasonably actually be concerned about AWS shutting you down.