Because the prices will be reduced for everyone. And since the price for everyone will be lower, matchday revenue will be lower and we won't take as much as we originally would so it's not as if we're somehow getting one over you. On the other hand, both clubs may even get the same/more since this game is sandwiched between far bigger games, all no doubt category A, and likely to feature weakened team/s. I would think this fixture is a much more attractive proposition to watch, now it is cheaper. As you say, the difference for the clubs is tiny, negligible, but the difference for fans is rather large especially when you take travel into account.
Your fans were calling for it too, so it's not as if this was some sort of demand the club made.
In any case, going by the Black Scarf people, all this does is bring it in line with your policy last season.