This is just 'the sky is falling' thinking.
If groupon defaults, the businesses will just not honor the groupon coupons. After all, they haven't been paid for.
The entire hypothesis of this article is that Groupon amounts to a marketing firm where you pay most of the cost in kind much later, instead of paying up front for a big marketing campaign. The 'in kind and later' part makes your scenario absurd.
However, I this article basically misses the point. Lots of businesses have excess capacity that costs them nothing to utilize. For example, a hair salon which employs 4 stylists will have several hours per week, and perhaps many more, where a stylist is idle. Giving someone a very cheap haircut when otherwise idle is a chance to win a future paying customer, and has basically no cost. It's a win for the customer and a win for the stylist, and of course groupon is getting a fee for that.
The same is true of items, like designer clothing and food, that have absurd markups. Selling it for closer to cost will cannibalize some future purchases, but overall isn't really that harmful, and may generate recurring income from newly converted customers. It is certainly MUCH lower risk than an advertising campaign, in that all the cost is baked in to people who actually show up in your store, instead of spread to the wind in the hopes of hitting the right people.
The problem with groupon is that it has no moat. http://37signals.com/svn/posts/333-warren-buffett-on-castles... Any competitor can come along and set up an identical business, and there is basically no network effect to speak of to keep customers coming to groupon instead of living social or any other competitor, and no risk for retailers or customers of trying another competing site. This is why there is 1 classified site in the US (craigslist) and 1 auction site (ebay), but innumerable comparison shopping sites all sharing the same retailers (amazon, half, cnet, shopping.com, pricegrabber, etc etc).