Yeah I have heard that some restaurants do lose out on those coupons. I would say if you feel guilty think about going back sometime. That's why they're doing it. It's advertisement, and they're hoping if you like it that first time you'll go back. The people that they don't like are the ones that only get groupons and stuff and go only to restaurants when they have them. They're not repeat customers and they don't intend to ever be. But, in the end, when you put up a coupon like that you take those people into account and hope you make out better through the people that do return or talk about your restaurant to a friend.
The company I work for uses groupon and living social to great success. We're a laser tag place, though, so we really don't lose any money at all when someone comes in with a groupon, because we don't really have some product that we're selling them. Sometimes they do take up space that a full paying customer might have used, but that's about it. Most people that come in tend to come back at some point, so we're not worried about it at all.