This discount is no more for two reasons - someone got the discount and told Facebook with the result that there were thousands of likes and shares. It's plain that a discount given because people chose to pray without prompt is very different from an offer - 'if you pray before eating you get 15% off'.
The other reason is far sadder and is that an organisation called the Freedom from Religion Foundation wrote threatening the owners with legal action under the Civil Rights Act.
"As a place of public accommodation, the Civil Rights Act requires the diner to offer goods and services, which we interpret to include discounts, without regard to religion, race, and national origin," says Elizabeth Cavell, a staff attorney at the Freedom From Religion Foundation.
What a sad, soulless piece of snippy legalese. It is action like this that gives atheism and humanism a bad name. Instead of smiling at what was essentially a selfless act by the owners (it clearly wasn't a promotion) we get threats of litigation. And this merely serves to remind me what a sad and crabby old world we live in.