I credit that Restaurant.com gift certificates do not have every characteristic in common with ordinary gift certificates purchased at face value.
That said, imagine a restaurant attempting to avoid honoring state consumer protection las as to gift certificates. "Our marketing agent sold you this piece of paper and called it a gift certificate, with our authorization by contract. But it was all a ruse: It wasn't actually a gift certificate; we just chose to call it that for our marketing convenience." I don't think so! Having chosen to call it a gift certificate, I believe they're stuck with that term, and all the consumer protections that then attach. In particular, having called it a gift certificate, I believe they're estopped from denying the consumer protections that apply to gift certificates.
I too had quite a negative experience at Annapurna: Owner refused to honor my Restaurant.com cert -- despite my pointing out that I had paid good money for the cert and probably wouldn't have visited his establishment without it.
Luther: You wrote: "legally speaking it is likely that nobody is strictly required to honor anything". I disagree. Annapurna entered into a contract with Restaurant.com when it agreed to let Restaurant.com promote Annapurna. We customers are clear third-party beneficiaries of that contract. Massachusetts consumer protection law is also on point. A restaurant cannot authorize an agent to distribute promotional offers, allow customers to arrive and incur expenses in expectation of beenfiting from those offers, then unilaterally refuse to honor the offers. No way!
I've visited dozens of Restaurant.com restaurants. This is the first that ever refusedt to honor a cert. Though the food met my expectations and the location is convenient, I won't be back.