I agree that's it's staggeringly arrogant to think that someone should have to explain their medical condition to you, so you can decide if they are deserving of the P & C space or not!
My friend who is registered disabled and has a blue badge, parked in a P & C space a while back, when there was no disabled parking spaces left, and was subjected to a tirade of abuse from a big nasty 30-something thug who had a baby with his girlfriend who was sat in the passenger seat...
Said girlfriend looked uncomfortable and kept telling him to 'leave it,' but he refused, and kept attacking my friend, telling her 'you don't look disabled to me,' and 'if you ARE disabled, it's you're probably disabled coz yer fat.' and came up right in her face and told her that her blue badge was a fake.
She refused to move out of the space and went into the store and told the security what had happened, who promptly confronted the man, and gave him a warning. He said if this happens again, he will be banned from the store.
My friend also used to get filthy looks from people over 60, for being in a disabled bay, because she looked too young to be disabled apparently. 
Some people are such entitled-to snowflakes. A P & C space is only a courtesy space, and there is no law attached to using them. No-one has any right demanding to know why someone is using them, and they should not be surprised if the person they confront tells them to fuck off.
As someone said earlier, if someone IS parked in them, they either don't give a shit, or they need it because of a disability. So why confront them? It's terribly arrogant to do so.