[McLovin]

So I'd be careful about running up a big bill and refusing to pay it with other than the gift card. You may very well have no legal grounds to do that.

If the restaurant is of any significant size (revenue) I'm sure there were lawyers involved on both sides of the sale transaction. My guess would be this was not an issue that went unnoticed or unanalyzed.

While what they are doing may not be the best move from a customer relations standpoint, I would certainly not assume it is not legally permitted. This thread is a good example of why a little knowledge of the law (and recommendations for conduct based thereon) can be a dangerous thing. If you are planning such a thing, I'd do whatever you have to do to ensure yourself you have the right to do it.