The legal definition of gambling in the United States depends on wagering some item 'of value'. I don't believe that the fact SD doesn't do the exchange matters - for a similar situation, exceptions to the law such as what you're talking about exist elsewhere in the world - see http://en.wikipedia.org/wiki/Pachinko. The fact that we don't have similar establishments in the United States would seem to indicate to me that something that can readily be exchanged for currency would count as having a value.