In general, the use of virtual currency is legal and fairly well-established at this point. The implementation and use of virtual currency will be governed by the sites terms of use (TOU) and/or end user license agreement (EULA).

These are contractual arrangements entered into between website publisher's and their user's and govern the use of the website and the virtual currency. The courts will defer to these governing agreements in settling any disputes between user's and the website publishers.

One could expect to see in a TOU or EULA language explicitly disclaiming any player ownership interest to in game items and currency on any site that uses virtual currency to engage its users. Thus if a user went to court and demanded any compensation the website publisher would merely have to point to this provision in the agreement to dispose of the claim.

In addition, a TOU or EULA may include an arbitration provision that could (depending on the jurisdiction and the prevailing caselaw) restrict user's access to the courts. User's subject to these provisions would be required to bring their disputes before an arbitrator.

The only time you would get an unexpected result is if your TOU or EULA is deficient or lacking, which highlights the importance of making sure these documents are in order.