If it is a common misunderstanding on the stores' part, I think it's Groupon's job to manage that. If an organization is set up to sell the product to the wrong customers, that organization should fix their sales process.

But I suspect it's a correct understanding on the part of the employees. When I talked with servers at restaurants, their experience was that the average Groupon customer was demanding, cheap, and a bad tipper. And they were also unlikely to come back, meaning there wasn't a lot of incentive to invest in a relationship.