How Does Groupon Work?

I noticed the "fine print"... the "promotional value" can be set to end at a certain date but the monetary value persists till used. That's some help I suppose. (I.e. if you pay $20 you still get that value towards normally priced food instead of the initial promotion.)

I think if I were running a restaurant, I'd only want the promotions to be good during times things would be slack anyway.