rhwbullhead raises a good point.

That gift certs are not supposed to get rewards is read so often on these boards that I, for one, assumed it as Truth. Now, I'm wondering where this idea comes from? I checked airline T&C and the various info pages on rewardsnetwork.com, and come up empty.

Also, a search of this forum reveals plenty of experiences indicating that they post just fine. That has been my experience, too.
Example thread on certs

Chain restaurant/gift cert thread

One thought is that the T&C notes that an individual restaurant also can impose additional terms, that will be spelled out on the details page about that restaurant.