It depends on the type of doctor, and the type of service. For example, dentists and oral surgeons use referral kickbacks almost 100% of the time. I've seen xray specialists and open-MRI centers also offer doctors kickbacks.

It's obviously not seen in HMOs and in emergencies or in large hospital institutions, but in the private medicine arena, where this guy is solely practicing, it certainly is more common than you make it seem.