You misunderstood completely. A doctor who isn't a psychiatrist cannot legally diagnose someone with a mental disorder, and therefore cannot prescribe a medication to treat said disorder. If he'd been prescribed the weed to treat pain or inflammation, it would have been okay.
It would be like a car mechanic recommending a specific type of weld to fix airplane wings. Yeah they're KINDA related, but he's still not an expert in that field, so it's not okay.
