They should have a sample seat next to where you get your boarding pass. If you fit into the seat with both armrests down and without spilling over onto another seat then you are fine. Otherwise you pay for 2 seats. It's as simple as that.

It could also be that places are giving people grief about using the Groupons in the first place, which could lead to the bad reviews. I had a restaurant say that I could only use the groupon for lunch, even though that wasn't stated in the terms when I bought it. A nail salon had a huge sign in their window that said that Groupons could only be used on weekdays, even though they didn't state that originally. I actually stopped buying Groupons because it was such a hassle to use them. The bad reviews could be people upset with rules and restrictions that were added by the restaurant/store after purchase, and not that they are overly picky people in general.