Nearly everyone tips based on the coupon value, not the market value. So that's maybe 50% less. They're also more "value minded" customers, who are likely to tip at the low end. Regular customers sometimes get the expensive bottle of wine; groupon users only ever get the cheapest bottle, if any. And then you get a subset of people who bring a $20 groupon and exactly $20 to the restaurant.

It doesn't mean everyone using groupon is automatically a jerk, but it definitely selects in that direction.

I naively thought that it's fair game that because I'm buying a £20 Groupon I might pay exactly £20. I never did, but I felt that it should be implied that that's acceptable. I certainly wouldn't have expected to be treated worse for it.

I think the expectations are not always in alignment, hence the friction. A simple solution would be for the groupon to say "$20, fixed menu, all tax and tip included." But you don't see a lot of that, although a lot of the advertising probably sounds like it. Your expectations aren't unreasonable, just... wrong. :)

> groupon users only ever get the cheapest bottle, if any.

Yeah, because absolutes like this help. Groupon customers can be just like any other customer out there. Some "regular" customers will get cheap wine, some will get an expensive bottle. Similarly, some Groupon customers will get the cheapest wine, others will take the money they saved by getting a Groupon and get the expensive wine. Being a "Groupon customer" might move the needle towards one direction but it's far from absolute the way you're making it out to be.

Many people tip on what they paid instead of the normal cost, so if they bought their Groupon at a heavy discount, they'll tip correspondingly less.