'You should not use the badge to allow non-disabled people to take advantage of the benefits while you sit in the car.'
This is not the same as 'you must get out of the car'. If that was what was intended, that's what would have been said, as it's much simpler to understand. 'You must get out of the car' doesn't allow for all the nuances of wanting and intending to get out, but not being able to, that have already been discussed.
'If a disabled person turns up with the intention of getting out but then can't and won't be, they can move to park in a normal space.'
But you don't necessarily know that when you arrive. All you know is that they can't get out right away, but they're still hoping they may be able to if they rest for a little. While they do that you don't have to stay with them, you can go to shop A, and then come back to see if they're up to it now. According to some people's interpretation, that's already misuse. Would it then become OK if the disabled person finds they can get out at that point? What if they're so exhausted by getting out that they can't make it to shop B? Would that be OK because they've got out? If they went back to the car to wait would that be OK or do they have to hope there's a nearby bench they can make it to?
In my reading of the rule all these situations would be OK because the disabled person has used the badge for themselves to try to go shopping. They have not used it to deliberately allow a non disabled person to take advantage. The non disabled person may sometimes get an advantage but that is incidental. It happened because of the disabled person's condition at the time, not because of anything they allowed.
To be clear, I completely agree that taking up a disabled space when everyone knows that the disabled person won't be getting out is the wrong thing to do. I also understand that if you've never had to face the reality of keeping some hope alive while living with an exhausting, progressive illness that drains you of everything, you probably think that some of the scenarios I've put forward are unrealistic. I wish they were.