You are missing the point of running a daily deal, it is not to increase demand so much as it is to increase exposure. Obviously no business could survive on these deals alone, which is why they are designed to only be for targeted periods of time and to targeted areas. The whole point is to get people in who are already in the market, but are tentative to try something new, or otherwise unknown to them.
Sure someone who buys a massage deal is probably going to buy a massage somewhere, but the beauty of running a deal is you have a better chance to expose that customer to your business instead of where they might have otherwise gone. The same can be said for any business.
Increasing exposure is the point. If this can be accomplished with no money up front(unlike traditional marketing) and with guaranteed usage(unlike print coupons)the business comes out ahead.
You are making the assumption that these businesses only have a set number of regular customers and no idea how to interact with any new customers. Trust that any successful business will know how to and want to grow their business. It is entirely up to them how they handle whatever influx of new business they receive as a result of running a deal.
Additionally, while I agree that there have been a few horror stories, in what way does this prove that the people purchasing these deals "would not have bought anyways"?
The fact of the matter is that the majority of businesses are happy with the end result of running these deals. Most gain a significant number of new customers which more than offset any possible losses they may have had in running the deal in the first place.
The businesses who haven't been happy were either exploited by their lack of research into this growth model, or didn't have control over their particular deal. The latter again being addressed by a few new deal sites out there, adility.com being the first.