It’s likely predicated on old monetary policies. Back in the day, the phrase on your paper money used to say “This bill is legal tender for all debts public and private and may be redeemed at the Treasury for gold or ‘lawful money.’”
You used to be able to turn in cash for gold or other commodities of value. Now our system of tender is just a house of cards.
Back in the '70s the tale was told of a guy who sent the Feds a $20 bill saying he wanted to redeem it for "lawful money". They sent him two $10 bills. He asked them how repaying one IOU note with two was legal. No answer. It didn't take 'em long to remove the "lawful money" quote.
[Sidebar I]: When I was a kid, the Big Snicker was to ask what a sailor looked for when on leave. When nobody could answer, we'd fold a bill with the above notation so that it said "gal tender and legal". A real knee-slapper.
[Sidebar II]: The old gold/solver backed currency were really just warehouse receipts that you could cash in any time. When I first read their legend, I swore it sounded like something coming down from the Mount. (Read the fine print at the top and bottom):