The reason the whip shouldnt deal extra damage on first glance is the fact that if you have a flaming longsword you cannot just make a touch attack with the weapon basicly trying to bypass armor to deal damage.
Where does it say that in the rules?
It has been the case (since 1Ed), for instance, that merely brushing flammables with a flaming sword will ignite them- you don't have to strike a parchment scroll, a pool of spilled oil, a bale of hay or a Web, for instance, to ignite any of them. Mere contact with the flame is sufficient to ignite flammables.