The question is why are people using the town centre car parks if the town centre isn’t open. They must be gaining some sort of benefit by being parked there, while the town gains no benefit as they are not spending any money in the town. So they should be paying for parking.
Now when the town opens up again, that is the time to remove the parking fees. Why charge someone for the benefit of coming into town to spend their money in the town centre shops and businesses. They should be encouraged to do this, not given a reason to go somewhere else.