You're not thinking of why restaurant owners like gift certificates.

It's because people who use them invariably spend more than the amount on the gift certificate (because it's hard to spend exactly what's there, and they don't want to "waste" a whole bunch of unused certificate), and so the restaurant gets more income.

The restaurant is much more interested in picking up customers (the users of the gift certificates, when they're used in the "normal" way) who will hopefully return repeatedly (this time paying real money), than they are in a one-time sale of gift certificates to someone out of the area, on which they will see hardly any income anyway (becuase iDine will swallow it all).

Restaurants aren't interested in miles addicts! They're interested in diners, especially local diners who then talk up the restaurant to other local diners who aren't miles addicts. (Remember, the restaurant pays very heavily for those miles; they generallly don't turn a profit on those dines, unless those dines result in more non-miles-earning dines. So they prefer only a fraction of their business be miles-earning dines.)

... Meanwhile, I don't understand why you're so interested in a random restaurant's gift certificates (unless you're going to find a way to use them somehow). Even with, say, a "double miles" promo (for an elite/tier 1), you're paying 5c a mile (at 20 miles/$ spent). That's a pretty horrid rate to pay for miles, if you're not also getting the value of a meal out of it.