Businesses that use Groupon do so because they want to find new customers and hope they come back. Treating these new customers poorly is bad business, and wastes the effort (duh).

However, many businesses get bitten by offering too generous deals with no limit on how many coupons can be sold, losing much more money than they had imagined. From what I understand by reading the news, it is possible to limit the number of coupons that can be sold, but this is discouraged by Groupon. And many, maybe even most, Groupon buyers seem to be cheapskates who never meant to come back at all but will swarm to whatever next fantastic deal comes along. Once the business owner finds this out, no wonder they are sore. So, the rude lady's behavior is at least explainable, though not excusable.