They are testing for abusive behavior to identify drug abusers, re-sellers, or negligent providers (over prescribing).
I have read some people complaining that their provider will not prescribe the drug they need if THC comes up in the test. That is not universal, it probably depends on the individual provider or the health provider system.
The best part is that law enforcement can review the records at will (as long as they can articulate an official reason which is a very low bar.)
True, but a positive test wouldn't be enough to arrest someone for drug possession. I doubt it would even be enough to get a search warrant with.
It tests for dozens of drugs, including THC.
All patients prescribed a controlled substance have to take it.
Here is a link to a random lab describing it: https://etd.paml.com/etd/display.php?id=8412
I was told that I "may need to" take this test at some point. I never have and have been taking adderall for many years.
I assume the drug testing requirements may depend on the state or the provider. It benefits the provider/health system by providing evidence they are taking reasonable precautions to limit abuse.
Also, I don't know if psychiatrists in WA are held to a different reporting standard than general practitioners. When I used a solo psychiatrist, no testing or privacy waiver required. When my insurance changed, I lost that psychiatrist and went to a regular doc in a large health system (UW Physicians). They require the testing and waiver.
But the law was changing when I made the change, so it may apply to psychiatrists now, too.