Disney has less problems filling the hotel rooms Fri, Sat, and possibly Sun night. It's much each easier for someone to decide to visit for the weekend, or take Fri or Mon off for a long weekend then it is to come midweek. I imagine that when they decided to change so rates apply per night, (no more checking in last day of value rates to get for the whole stay) that it made sense to do the same for the AP rates. Simple matter of demand. Same reasoning, but opposite days, for why rental cars are so cheap on the weekends. (Less business people using them while on trips)