About the price increases on AP items

I might be a little biased about this, me being a mage main and all, but why did the cost of most of the core mage items go up the same patch that ADCs got a huge buff? I don't feel as though mages were so dominant before that deserved this nerf. Particularly harsh is the 500 gold price increase to Zhonya's Hourglass. I've been maining kennen for most of the year but he simply doesn't work as well anymore because hourglass is so essential to the champion. What gives?