You're right, your dentist does have a contract with the insurance company and so has agreed to see patients that the insurance company sends his way. I was referring to the more general relationship between a dentist (or other healthcare provider) and patient.
It also sounds like there was more to it than you just saying, "No thanks, I'd like you to do less work than you were planning on." I would not be at all surprised if your dentist anticipated that you would be hard to satisfy in general (not just in this instance), and having dealt with that before, he would just rather not provide treatment. He may be wrong in that assumption, but he's going off his own experience. Since you insisted that he provide services below the standard of care, if he's not comfortable with that, he also has a legitimate reason to ask you to seek dental care elsewhere.
For the person who wanted to know about radiation exposure, the effective radiation exposure of a full mouth series (usually 18-20 x-rays) using the least sensitive film (therefore most radiation exposure) is 388 microsieverts, which is equivalent to 47 days' worth of natural background radiation. The effective dose of a set of bitewing x-rays (two on each side of the mouth) using the most sensitive digital sensors is 5 microsieverts, roughly equivalent to half a day's worth of background radiation. Those numbers are based on calculations from the International Commission on Radiological Protection.