Another aspect of "Legal Tender" laws not often discussed, is that from a Monetary Theory perspective you cannot use debt to pay debt.
So if I owed you "3 silver pieces", I could not in pay you with an IOU for "3 silver pieces" that someone else had given me.
Due to the nature of a FRN being in theory redeemable for USD (but not the same), they had to pass a law stating that FRN legally represents the real thing (ie. a real USD).
Of course just because they pass a law that says "a circle is now a triangle" doesn't make it true, and the follow-on ramifications of this is that now all debts are "discharged", not actually "paid in full". So the restaurant agrees to discharge your debt but you haven't actually paid them.
All of this stems from the Bankruptcy of the USA proceedings in 1933, from which we have never technically come out of Chapter 11.
The full text of HJR-192 is an interesting read, here it is below:
To assure uniform value to the coins and currencies of the Unites States,
Whereas the holding of or dealing in gold affect public interest, and are therefore subject to proper regulation and restriction; and
Whereas the existing emergency has disclosed that provisions of obligations which purport to give the obligee a right to require payment in gold or a particular kind of coin or currency of the United States, or in an amount in money of the United States measured thereby, obstruct the power of the Congress to regulate the value of the money of the United States, and are inconsistent with the declared policy of the Congress to maintain at all times the equal power of every dollar, coined or issued by the United States, in the markets and in the payment of debts,
Now, therefore, be it Resolved by the Senate and House of t Representative of the United States of America in Congress assembled, that
(a) every provision contained in or made with respect to any obligation which purports to give the obligee a right to require payments in gold or a particular kind of coin or currency, or in an amount in money of the United States measured thereby, is declared to be against public policy; and no such provision shall be contained in or made with respect to any obligation hereafter incurred. Every obligation, heretofore or hereafter incurred, whether or not any such provision is contained therein or made with respect thereto, shall be discharged upon payment, dollar for dollar, in any coin or currency which at time of payment is legal tender for public and private debts. Any such provision contained in any law authorizing obligations to be issued by or under authority of the United States, is herby repealed, but the repeal of any such provision shall not invalidate any other provision or authority contained in such law.
(b) As used in this resolution, the term 'obligation' means any obligation (including every obligation of and to the United States, excepting currency) payable in money of the United States; and the term 'coin or currency' means coin or currency of the United States, including Federal Reserve notes and circulating notes of Federal Reserve banks and national banking associations.
Sec. 2 The last sentence of paragraph (1) of subsection (b) of section 43 of the Act entitled 'An Act to relieve the existing national economic emergency by increasing agricultural purchasing power, to raise revenue for extraordinary expenses incurred by reason of such emergency, to provide emergency relief with respect to agricultural indebtedness, to provide for the orderly liquidation of joint-stock land banks, and of other purposes;, approved May 12, 1933, is amended to read as follows:
"All coins and currencies of the United Stated (including Federal Reserve notes and circulating notes of the Federal Reserve banks and national banking associations) heretofore or hereafter coined or issued, shall be legal tender for all debts, public and private, public charges, taxes, duties, and dues, except that gold coins, when below the standard weight and limit of tolerance provided by law for the single piece, shall be legal tender only at valuation in proportion to their actual weight.'
Approved, June 5, 1933, 4:40 p.m. 31 U.S.C.A. 462, 463
House Joint Resolution 192, 73d Congress, Sess. I, Ch. 48, June 5, 1933 (Public Law No. 10 )
