They have to check. One of my vivid difficult memories growing up was of my grandmother who did have legitimate pain issues but who didn't communicate to all of her various doctors what all of her prescriptions were. I don't think she was drug-seeking, as much as she just didn't think to do so. Well, she was having a knee replacement surgery and didn't tell her doctors all the needed information, she wasn't given all of her usual pain meds in the hospital because they had no idea, and she went into severe withdrawals. I was about 13 and walked into her hospital room to find her in a raging psychotic break. After that incident, we knew we had to go to her appointments with her and be sure all of her information was given to all of her doctors; we just didn't realize the need to do so previous to that. We had no idea what was going on.

I suspect this is a very common scenario where people don't give their doctors all of the information for whatever reason, and it can result it in very dangerous situations.

They have to monitor what is going on. They have to figure out if what they are seeing in a patient's behaviors is a psychiatric issue, an illness issue, a med issue. Oftentimes even the patients don't realize what is causing what. So drug testing is a necessity, even in patients with legitimate pain issues.