From Disney's website:
"Effective for reservations made March 21, 2018 and thereafter, standard overnight self-parking is available to registered Guests for a fee that will be applied to their hotel folio upon check-out. All parking fees include applicable tax."
Depending on how you read this, they may mean for Reservations made FOR March 21 and thereafter. Or they mean for Reservations made PRIOR TO March 21 and thereafter.
If it is the prior, they may be charging everyone after March 21, and they don't need to track when you made your reservation.