I really would like to see what was the legal reason behind that, I know the US has issues with gov't agencies using their opinions as law but I thought mail was constitutionally protected?
I'm glad that Twilio fought for Title II governance: https://ecfsapi.fcc.gov/file/60001324418.pdf
Where was everyone else?
As far as the sanctity of the U.S. Mail, it only applies to sealed envelopes/packages, and Congress can ban items from the mail (e.g. lottery pamphlets, spurious tokens, gasoline, etc.)