Because people who haven't played heroes much are clearly bad with them by default, so its your chance to play AM that you aren't normally brave enough to try, even though you know how to play him, and stomp faces!
Stupid game mode, imo, the logic "not played a hero = bad with it" only applies to the average player. Considering that's the majority of the game, I don't fault them for making the game mode, but I will never use it myself.