The goal of this change is mostly to fulfill the basic promise of Vayne / Tristana. At level 18, these characters are supposed to absolutely destroy the enemy team - so long as you can get them there. They are, along with Kog'maw, some of the truest ranged carries in League of Legends at the moment.
However, with the decrease in power from itemization - these characters are suffering from a mid-game slump and a farming slump - thus power in their levels seemed to make sense for addressing their basic curve. This change basically ensures the power level of these traditionally weaker early-game carries for guaranteed power late game.
As for Ashe, she's kind of the ultimate late game utility/kite carry - thus the attack speed here is mostly so that she is always able to fulfill her role as kiting carry. Attack speed is a major factor in being able to "orb-walk" to poorly use DoTA Jargon (Attack Move fluidity, roughly).
Amid the turmoil and tumult of battle, there may be seeming disorder and yet no real disorder at all Sun Tzu