The FCC just makes decisions, we're talking regulations here, not laws. They justified it with "preventing spam" and enabling competition with unregulated OTT apps.
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.)
https://supreme.justia.com/cases/federal/us/96/727/