I think they are doing the only reasonable thing they can do under the changed circumstances. When they offered this promotion, they expected a June 1 "back to normal" and it just isn't possible now, which has a major impact on the availability of this promotion. Think about it: the promotion was offered for people whose March-May vacations were cancelled, and was for any dates June 1-Sept. 30. Now that June is cancelled, if they allowed this promotion to continue to be booked/modified, everyone who rebooked to June would move to July-Sept. And what if July is cancelled to? August-Sept would end up massively overbooked with free dining participants, on top of people who had already planned vacations in August-September and already made dining reservations 180 days before their trips. Not only is that a catastrophe for restaurant availability; it could also present a real problem for park capacity limits (which will almost certainly be restricted for some time, like in Shanghai).
This offer has been on the table for over a month now, and they are letting people who booked it keep it--that's first come first serve, and under the circumstances, really the only fair thing they can do. They are going to be hard pressed to guarantee park and restaurant access for everyone who currently has a resort booking in those months as is--is it really any surprise they don't want more people booking with a heavy discount right now?
As for "providing an explanation" would that really make a difference? We all know what the explanation is; just like when they announce a ticket price increase, they don't have to explain why--it's supply and demand, just like this decision.