> 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. Foreign gold or silver coins are not legal tender for debts.
They argue that the fees at the parks are public charges, not debts. I can't find anything in the US code that provides a definition for "public charge." It may be there and I just haven't found it. It seems likely that there are prior judicial cases that help shine a light on what is and is not a public charge. I hope these folks win.
Public charges seems to include things like fees for services and tolls, so it would seem that payment for entry to public parks is included here. Unfortunate though, as it may not make financial sense to open up areas if it requires someone there to accept payment (unless enough people are paying on an honor system). It may also result in parks just widening the scope of specific park passes and increasing the cost of a pass, meaning you can no longer visit one specific area of the park for a small fee. I wonder if you could just require that cash payments be made in advance at a park center if you don't choose to pay online.