Scorched Earth would make more sense if it reduced the bonus to 0. Attacking an area that has been "scorched" shouldn't potentially kill of the entire attacking army. It would possibly make more sense for the blockade card.
Same could be said for wastelands. As I see it, 'armies' are an abstract concept anyway, and in the case of wastelands, they are used up to restore the land to productive state. Same would be the case for scorched earth lands. You use up 'armies' to ruin the land, and the enemy uses up armies to fix it again.