I've had this experience a few times and every single time it was mentioned on the Groupon, I actually checked out the menu before hand, and bought or refused the deal there and then. This isn't really unique, you buy a specific deal so you get that deal's menu, Groupon rarely sells some kind of carte blanche 20% discount card on whatever the company offers which seems to be what you were expecting.

I've also had Groupon redeem me for stuff that I had no right to in the TOS but was morally sensible to do.

There are quite a few things I criticise Groupon for but they've always been pretty transparent about their deals and willing to right any wrongs in my personal experience.