I thought the illegal action was the "deBASEment" of the currency, not defacement. When coins were made out of precious metals, they could be shaved for bits and slivers of that silver or gold. Since the coins weighed less, but still represented the amount of money it was promised to by the government, the currency was debased. And that was a major crime.
It defacement of the currency is illegal, then we would've locked up all those wheresgeorge.com people, who keep stamping one dollar bills.