Doctors, like most professionals, charge for their time on an hourly basis. Most doctors, though, set up their bills so you are just charged a flat rate for an office visit; however, that fee is based on an estimated time that he'll spend with each patient and then calculated out to determine what he wants (or needs) to earn.
Thus, when the doctor is spending more time with each patient (performing services that the patient doesn't request), then the fee for the visit needs to be higher so the doctor can continue to cover his nut.
Don't think your time with the doctor is free. It's not.
LOL...they have to "code" their billings, unlike lawyers. Although it was lawyers who came up with the idea.
That's why surgeons make so much more (if you go by hourlies) than a subspecialty internist, but their yearly salaries work out very similar. Doctors can charge by procedures, like sewing up an idiot hunter who got drunk and fell out of his blind, but he can't charge extra for recommending that he get a tetanus shot if the hunter's going to be outdoors and stupid at the same time.