While I do not doubt that saving cash is a factor, it is my understanding that guests in WDW actually complain about the overlays. To make matters worse, they don't appear to result in increased turnstyle spins like they do at the other parks.I thought there had been a hint they would but that hasn't seemed to happen yet.
For a park that charges for a Christmas party they could afford to do a few overlays, like Small World and Country Bears. They get away with Jingle Cruise ...