Doctors, teachers, priests, bureaucrats, politicians (the list goes on and on... managers, cashiers, mechanics) are apparently used to the authority that comes with their position.
Through my life I've been eager to please and respectful of authority. Polite. I've bent to make the transaction easy.
It's true that you can negotiate best when you can walk from a deal. You can walk from a deal when you're in control and know what you want. and, better yet, know that you can live without the deal.
When authority is complacent, merely counting on respect and ignorance (theirs are the expert opinions we must trust and follow, aren't they?), they are in a position of weakness. It's amazing to watch their faces when you take control of the situation by weighing their input and politely telling them "No, I'm not going to do that."
It's like a cat with a mouse.