Good question. Being able to book from outside the parks is surprising.
If you can book from outside the parks, that could prove problematic as this moves forward. At the very least I hope the ticket has to have been scanned for entry before it can be booked on that day.
GPS would be one way. Like even the Mousewait app checks your GPS location when you try to submit wait times for a certain ride. If DL/DCA ever get half decent wifi, like a full outdoor mesh, then they could get really accurate.