Barb Whirlwind not procing 'On Melee' Effects

I've noticed that "On Hit" effects proc any time an enemy is hit with a spell of any kind. I guessed that 'On Melee Attack" would be similar, but only when the attack is made with a melee weapon. This seems to be true since the axe procs off spells such as HotA and Seismic Slam, though they still have AOE attributes caused by a melee swing. However the axes do not proc when using the Whirlwind ability, is Whirlwind not considered a melee attack?