If you look at the icon on the card that makes your opponent remove a legion, you see that it crosses a legion on a circle.
It's the same iconography as on the objectives that allow you to place legions onto certain objective icons. In the same way as these bonuses are designed to speed up completion of objectives, the aggressive action that removes legions from objectives is designed to slow down your opponents' completion of objectives, the difference is that speeding up is 1 objective, slowing down is one objective per player. So not that lame.
EDIT: Also, regarding iconography, if you compare the icon of the legion that is removed, with the one that gives a player more legions to play with, the one that gives is again missing that circle on which the legion stand on cards.
The world now makes sense. thanks