Is this in anyway based on any facts whatsoever or is this something you weer once told and are now repeating? A quick Google says it's up to 5 years jail for keeping misdelivered mail in the USA (depends on jurisdiction).
https://www.hg.org/legal-articles/theft-or-receipt-of-stolen...
Yes. https://about.usps.com/publications/pub300a/pub300a_v04_revi...
Incidentally, we've been having numerous postal issues at my complex because it doesn't have a full-time carrier assigned to the route. It's so bad, I had to get a PO Box.
My neighbors must be mostly thieves (keep) or too lazy (throw away) to bother correctly redelivering mis-delivered letters or packages most of the time. They are surprised like you're a fool for making an effort to redeliver them correctly. The complex is 98% white-collar, third-tier college graduates, single, between 24 and 35. I've had electronics, clothing, and random articles left in my box and at my door... these are mis-deliveries.
Misaddressed mail would be wrong name but correct address.