Up until the 1960s the comstock laws were still in place, which made it illegal to send "obscene" materials through the mail. It was also illegal to send contraceptives, and or sex-ed materials, materials relating to homosexuality, etc.
Originally Posted by Photo Engineer
I would be so proud to say that we're much smarter now than we were in the 1870s, but unfortunately I know a good deal of contemporary Americans would love nothing more than to hurl the united states more than a century backwards legally.
In any event, if would have been illegal in the 1950s to return the finished prints or developed negatives by mail. Perhaps it would have been legal for them to be picked up in person, but I don't really know.