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.)