Off topic, but this reminds me very much of being put up in a hotel by the airline Avianca when they oversold a flight I was checking in for, meaning I had to take a different flight the next morning.
We were told that dinner would be provided, and given a coupon for the hotel's restaurant. Upon presenting said coupon prior to being seated, we were greeted with a look that I can only describe as 'oh, I see' - promptly seated at the absolutely worst table in the place, and given a 'special menu' with one or two very plain options (think, chicken with vegetables and a plain sauce) in an otherwise very nice-looking restaurant. We were summarily ignored for a very long time before our order was collected, and when the food was served it was sort of plonked down with no ceremony or pleasantries.
It's an awful experience to be treated this way, as a second class customer. As though you've broken a social contract by being allowed in through some hack, and the staff will 'serve' you to the absolute letter of the definition and no further. Almost pointedly so as though to emphasise how unhappy they are with your presence.
I never used groupon for a restaurant, but I can completely see that something like this might happen and understand why you never went back!