There is actually another thread active right now about the exact same topic (
http://disboards.com/threads/has-anyone-else-been-told-this.3403073/), as I explained in my post on that thread, this is likely due to a minimum length of stay restriction, this is common in the hotel industry, not just Disney, what appears to have happened is that Disney has just recently jumped on board with the rest of the industry, my guess would be due to excessive amount of changes and cancellations that were resulting in lost occupancy and profits over shoulder dates.