The problem dates from the early 1930s, when utility companies delivered their bills by mailbox through their own employees out reading meters. The Postal Service saw a huge drop in revenue and decided to act. Enforcement became even more important in 1970, Dalton wrote, when Congress mandated that the Postal Service be self-sustaining.
From http://latimes.com/editions/orange/la-000000534jan03.story?coll=la%2Dedition...
Postal Service Delivers Stern Warning on Mailbox Misuse Enforcement: Agency tells Orange mobile home park residents to stop using receptacles to send notices, rent bills, newsletters sans postage. ..... Q: When government acts like this how are they different from a gang? A: Both rule by violence and terror, but a gang claims no moral authority.