Article I, Section 10, Clause 1 says “No State shall... make any Thing but gold and silver Coin a Tender in Payment of Debts...” since the "United States of America" is not a single STATE, but a union of States, it is said that it doesn't apply. And they also claim this doesn't apply, since they are not making "coin" but paper "notes".
If you see "goldbacks", they look like dollars, they are literally gold "coin" produced for various States. True constitutional money that cannot be made illegal because of it's clear adherence to the above clause in the constitution.
So very interesting they found a loophole or so they believe…..🙄