Why is it illegal to stamp "Past Due" on an envelope if the contents are actually past due?

It is illegal for a third-party collection agency to do this, but not for the creditor himself.

(source)

@thorninmud is right. Magazine subscription departments, and even charitable organizations, would routinely send letters disguised as dire invoices in an effort to defray that unopened letter from the trip to the waste can. When I was in college, a very creative and funny friend got her hands on a big rubber stamp from a fireworks distributor. One night, under the influence, she grabbed a box of envelopes and stamped both sides of about a dozen of them “EXPLOSIVES” in big red letters. She then addressed them to friends stamped and mailed them, staggered home and passed out. The empty envelopes were all promptly delivered, though one was apparently delivered to the wrong address, opened, resealed marked “delivered to wrong address”. It showed up at the proper recipients’ a few days after the others. But those were the days before anthrax and 9/11.

Did not know. It always annoys me when I get a past due envelope for some charity thing. They still do it. Another one is when they stamp Immediate reply required in red and then have it from some political polling place but they use a government name that makes it look official and seem like you are in some sort of trouble.