Where does it say that?
https://www.usps.com/send/can-you-mail-it.htm?showtab=2
you can mail money, you can't enclose it in an envelope. they just won't take it. i mean, try it.
What if you didn't enclose it in an envelope and put the address and stamp right on the money?? Try to see USPS work their way out of that one!
haha.
Defacement of Currency
"Defacement of currency is a violation of Title 18, Section 333 of the United States Code. Under this provision, currency defacement is generally defined as follows: Whoever mutilates, cuts, disfigures, perforates, unites or cements together, or does any other thing to any bank bill, draft, note, or other evidence of debt issued by any national banking association, Federal Reserve Bank, or Federal Reserve System, with intent to render such item(s) unfit to be reissued, shall be fined not more than $100 or imprisoned not more than six months, or both."