Is It Normal for Clinics to Pay Patients for Referrals?

I had a Hair trasplant procedure 5 months ago and i recieved a email from the clinic which read;

By now you should be starting to see some of the results from your procedure with us. Please forward pictures for your file, and any questions you might have. What type of hair products are you currently using? Don’t forget our referral program, we will pay you £500 for every patient you refer that has a procedure with us.

My question is, is it normal for a clinic to pay a client money if they refer a patient to there clinic for a procdure.

This may be different depending upon the country, but to my knowledge it is illegal in all of the United States. Despite this, the practice is unfortunately common not only for patient referrals, but for hair dresser and other stylist referrals, though if a doctor is caught, he/she could lose their license to practice medicine.