Shik, re-read pages 6 and 7 of this doc:
Lawful Money
I think there may be a distinction between "lawful money" and "lawful money of the U.S." That distinction being who issues it. Both may be lawful (authorized by law) but one is issued by a private bank and the other by the Treasury. I don't know for sure, but it seems to fit. At any rate does adding "of the U.S." make it any less effective?