Medical patient/doctor ethics question for medical/legal pro's -?

I'm on several RX's from my family physician, a couple of which are controlled - when I went in the other day to pick up an RX, which I do routinely, the nurse called me in the back and said they were giving all patients a drug test (urine) to see whats in their system. I guess the idea being (1) to look for things that are Not prescribed, and (2) to look for things that Are prescribed but Not present in your system. I am just curious - for any of you medical professionals or legal experts out there - is this ethical and legal for a physician to do surprise random drug testing on his patients? Is there some new law I don't know about that went into effect? -Just confused about what started this, and frankly it was offensive as it makes you feel like some kind of criminal going thru an arrest process or something. Thanks for any input from people who know about this procedure.