If this money were for selling a product at competitive prices, there would be no objection. But it wasn't. This money was derived from preventing sick people from getting health care - patients were treated as cost items, health care was treated as a corporate loss, and the real losers were patients. The doctors get involved because they're losers, too. They get to sweat at the defense table during the malpractice trial.
Unions negotiate remuneration, hours and working conditions.
The main thrust for doctors to form unions is deteriorating, demoralizing and insufferable working conditions in which clerks who can't tell the difference between peripheral neuropathy and radiculitis have the effrontery, no, the buffoonery, to tell skilled specialists what's medically indicated and why whatever the doctor advises is a noncovered benefit anyway. The corporate entity is in business to create shareholder profit. This objective is not in tune with companies that seek to provide health care in which patients are supposed to come before profits. This method works neatly to disenfranchise areas of town where the proportion of sick people is higher. In one city, a group of doctors was told that if it provided coronary stents for its managed-care patients, the cost of the procedures would be deducted from the doctors' salaries.