They are testing the system. They could have easily chosen people who booked through Disney on the phone; or they could have selected people who booked directly through Disney online; or they could have selected people who are only staying at moderate resorts; or so on. They chose to use DME. They picked a group they believed will spend all of their time on property, and DME users are the most likely people to remain on property. This isn't rocket science.
I understand that they are currently testing the system, and I understand that DME+OnSite may or may not be the final decision on how you get FP+. I'm just saying that, if it is, then they are going to need to expand the airports that DME goes to.