I have seen it mentioned in here a letter from IRS David put up showing that when someone demands Lawful Money is telling them the person is not in the FR system.
Another thing I have not found yet is the connection through on this statutorily, lets take a look:

Title 18 Section 8 defines a FRN as an "obligation of the United States."
Section 8. Obligation or other security of the United States defined
The term "obligation or other security of the United States" includes all bonds certificates
of indebtedness, national bank currency, Federal Reserve notes, Federal Reserve bank
notes, coupons, United States notes, Treasury notes, gold certificates, silver certificates,
fractional notes, certificates of deposit, bills, checks, or drafts for money, drawn by or
upon authorized officers of the United States, stamps and other representatives of value,
of whatever denomination, issued."
Title 31 Section 3124 states that "obligations of the United States are EXEMPT FROM
TAXATION BY A STATE."
"Section 3124. Exemption from taxation
(a) Stocks and obligations of the United States Government are exempt from taxation by
a State or political subdivision of a State. The exemption applies to each form of taxation
that would require the obligation, the interest on the obligation, or both, to be considered
in computing a tax.


I am sure others will have something to add here, it would make for good edification with new people in the movement on this forum.