In other words they've just privatised the mail service.
If they have a state sanctioned monopoly you legally just can't.
n 1844, hearing from citizens from every party and under pressure to reform the postal system, Washington lawmakers and the Postmaster General had no intention of sitting still for any of "that Spooner's shenanigans." Suits against Spooner and his cohorts began. Railroad heads were given full warning that contracts for government mails would be removed and fines imposed unless space and passage were refused to private letter carriers. It was "round one" for the government when an agent of Spooner's company in Baltimore was found guilty and fined for transporting letters in a railroad car over a post road of the United States. Spooner himself was arrested in New York on March 7 on three charges by special agents of the Post Office. Another of his agents, Calvin Case, was held to bail for $100 around March 23 for carrying letters on the train.
https://www.pennypost.org/pdf/penny-post-archive/PennyPost20...