Whether the 50% lines work for people is on a case by case basis, and you're certainly not alone in thinking there are some serious problems with it.
However, if you think of it another way, Groupon is marketing. Sure, using Groupon costs them money, but so does typical marketing, whether it's a clever PR campaign, or a TV advert, or a spot in the local newspaper... The real question is always going to be whether the results Groupon delivers you are worth the cost of the campaign, and how that compares to alternative forms of marketing.
However, if you think of it another way, Groupon is marketing. Sure, using Groupon costs them money, but so does typical marketing, whether it's a clever PR campaign, or a TV advert, or a spot in the local newspaper... The real question is always going to be whether the results Groupon delivers you are worth the cost of the campaign, and how that compares to alternative forms of marketing.