(Edit: This is incorrect. See comments.)
(Edit: This is incorrect. See comments.)
IANAL either, but I would be interested in a citation on that. My understanding is that say in the US (a) you can't print US dollars (because the Fed has a monopoly on that) and (b) you can't refuse to take US dollars (because they are legal tender). That said, I don't know of a law that says you can't also use an alternative currency. For example if a school had "school money" where "credits" are earned for good grades and are exchanged for lunch or snacks, that is a form of alternate currency that surely is not illegal.
Federal reserve notes have the very handy attribute in that they are the only currency that can be used to settle U.S tax liabilities.
Start treating those casino chips as currency and the government will step in and stop it.
That was one of the things Neal Stephenson got right- you almost certainly need to hide inside a non-US sovereignty.
The issue with alternative currencies is primarily that of tax evasion. Income earned in any currency/denomination is taxable, and most users of alternative currencies underreport or do not report their income. This problem is especially prevalent where the currency's value is not tied 1:1 with the US dollar.
"Congress has exclusive power to coin money in the U.S. and to regulate its value, according to the Treasury Department."