and part of defending those institutions is punishing bad behavior, regardless of how much you might agree with it or think its righteous.

Because the carrier does not get to dictate who gets what mail. Their job, the entire basis of the institution, is to deliver the mail on their appointed route, regardless what it is, regardless to whom it is to.

You arguing that each postal carrier has some intrinsic right to not deliver mail they find objectionable is arguing for the destruction, not the defense, of the US Postal Service.