Or casinos. Both with the chips themselves and Player's Cards. Or Frequent Flier miles, which are so much of an alternate currency that the IRS taxes them at a certain point.
Alternative currencies are fine, as long as they are not being represented as US currency. Most of the private currencies, including casino chips, are merely tokens of private contracts between the holder and the issuer.
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.
