The reason the car parks are free now when next to nothing is open is probably because so few cars are using them that it would cost more in revenue to pay someone to patrol the car parks and collect the change/maintain the ticket machines than they would collect in parking charges.
Especially when nobody would be prepared to pay any charges whatsoever, so the car parks would be even emptier again.
Makes more sense to open up the car park to the community free of charge and therefore provide a civic benefit than make life more difficult during times of Crisis.