No different than gift cards (which are really private currency.)
Except a gift card holder must hold assets and honor the exchange value of the card’s terms. The issuer of the Liberty Dollar, for example, is making the claim that the paper gold and silver certificates are backed up by their hard asset. If they don’t have the hard assets to back up the value of the currency(sic) they are selling, they run afoul of securities laws. Unlike a gift certificate at a store, however, the claims they are making put them more on the level of banking regulations, and thusly, would have an entire larger set of regulations they must meet.