I'd never heard that reusing a stamp is against the law so I just did a quick search. Sure enough it is.
https://www.law.cornell.edu/uscode/text/26/7208
Here's the part that shocked me:
"shall be guilty of a felony and, upon conviction thereof, shall be fined not more than $10,000, or imprisoned not more than 5 years, or both."
Can you imagine sitting in a cell block as prisoners ask around "what are you in for?" Rape, arson, murder, assualt... reused a stamp. LOL
Has anyone ever actually been punished for reusing a stamp? Seriously.