While the bills may say "good for all debts public and private" there is no law that says any business has to accept them. Because before there is a bill there has to be some kind of agreement to establish the debt. Maybe a few states have recently passed laws. but those are going to be hard to enforce and prosecute. I doubt it is a felony, the courts in many places are already overwhelmed, the merchants have more legal and financial resources than the vast majority of those who only have cash etc etc. If I had a store and someone turned one of my clerks in the legal answer after stalling as long as possible would be they believed it was counterfeit.
The U.S. Treasury answers this question of legal tender acceptability on their website thusly:
A: The pertinent portion of law that applies is the Coinage Act of 1965, specifically Section 31 U.S.C. 5103, entitled "Legal tender," which states: "United States coins and currency (including Federal reserve notes and circulating notes of Federal reserve banks and national banks) are legal tender for all debts, public charges, taxes, and dues."
This statute means that all United States money as identified above are a valid and legal offer of payment for debts when tendered to a creditor. There is, however, no Federal statute mandating that a private business, a person or an organization must accept currency or coins as for payment for goods and/or services. Private businesses are free to develop their own policies on whether or not to accept cash unless there is a State law which says otherwise. For example, a bus line may prohibit payment of fares in pennies or dollar bills. In addition, movie theaters, convenience stores and gas stations may refuse to accept large denomination currency (usually notes above $20) as a matter of policy.
So, if a business states up front "no cash" you can not engage them in any transaction and then try to settle it using cash.
I am a cash guy and usually carry several hundred and sometimes considerably more if I am looking to make a deal on something.
Twodog Max is 100% correct about some "civil servants" who will impound your cash claiming it is drug money and making you prove otherwise. That is total ****, if I want to go around with a suitcase full of bills that's my business and according to the 5th amendment I can not be deprived of it without due process. I understand the why behind it but it is still sometimes used as nothing but "legal" robbery.