It's even the opposite: the US Postal Service is *positively obligated* by the 1st Amendment to deliver content without discrimination. E.g. Lamont v. Postmaste

It's even the opposite: the US Postal Service is *positively obligated* by the 1st Amendment to deliver content without discrimination. E.g. Lamont v. Postmaster General (1965) (US post can't create friction for US citizens subscribing to Soviet propaganda newspapers by mail). https://en.wikipedia.org/wiki/Lamont_v._Postmaster_General https://en.wikipedia.org/wiki/Lamont_v._Postmaster_General (It's also prohibited from opening mail without a warrant, but that's an orthogonal question).