That it is the defensive secretions of the amphibians that the snake is keying on as a potential prey item. I doubt that is true (otherwise, they wouldn't be stimulated to eat an amphib that wasn't alarmed). What chemical it is that triggers the feeding response in the snake would be an interesting study. How you would isolate it to study it is another issue!
I have to agree that Ranid frog scenting doesn't work nearly as well as toads, treefrogs, or spadefoots (spadefeet?). It may be that ranids are big, fast, powerful frogs that snakes don't bother trying to catch and subdue? Even though toads are large, they aren't that hard for a snake to catch.