1. The merchant didn't think through the consequences of their promotion. They made a deal that was too appealing to an unnecessarily broad swath of potential customers. A commenter on the blog points out that the better approach is to figure out how to bait the hook for a specific type of desirable customer instead of having a fire sale.
2. Groupon didn't look out for their partner merchant. Groupon's job is to be really smart about the business they're in and to share those smarts with their partners. Sure, they're relatively new at this too, but a part of their sales process should be qualifying the deals they're going to be running for people and saying "hey, you know, this might be giving away the farm."
3. People are dicks; doubly so in a down economy.