Dressed In A Little Brief Authority

We parked outside the dentist's yesterday afternoon- on double yellow lines. We had our disabled parking permit properly displayed in the front window and were fairly sure this trumped the yellow lines, but just to be absolutely certain we asked the traffic warden who was standing there for his opinion. He burbled something that seemed tantamount to giving us permission. Twenty minutes later we left the dentist's to find we'd been ticketed.

Surely the job of a traffic warden is to stop people parking in prohibited areas, not to lure them into breaking the rules so punishment can be administered? What is it about dressing a person in uniform that turns them into an enemy of the human race?