From what have found working in restaurants is that whenever there is a sale or deal it brings in a different kind of customer. They are usually more rude, more demanding, and less satisfied.
For instance when I worked at Subway and they would do 5 dollar footlong month, the customers that came in would want on average twice as many vegetables and sauce on their sub than normal customers, would complain that extra cheese and meat wasn't free and would usually end up being more rude then the regulars who come when there is no deal.
If groupon brings in this same class of people who possibly want as much as they can get for as little as they can pay it would make sense that they would leave bad reviews that normally the restaurant would not receive.
For instance when I worked at Subway and they would do 5 dollar footlong month, the customers that came in would want on average twice as many vegetables and sauce on their sub than normal customers, would complain that extra cheese and meat wasn't free and would usually end up being more rude then the regulars who come when there is no deal.
If groupon brings in this same class of people who possibly want as much as they can get for as little as they can pay it would make sense that they would leave bad reviews that normally the restaurant would not receive.