I guess until your best friends dead mate starts calling him up on the telephone, you don't know or think to investigate the full range of psychological problems that it can bring out. And many people don't know they are predisposed to a mental illness until it's too late.
I think that the psychological issues are well worth mentioning because drug induced psychosis is a real risk for some users but many other popular drugs that are legal also have psychological side effects. I know of a woman who had psychosis caused by her implanon, the weight loss drug (can't remember its name) can cause huge issues and so on.
The reality is that people know it is effective so many people who have never used it before turn to it for medicinal reasons. If a drug is legalised as a medicine and administered by doctors that means that Drs can also keep an eye out for psychological issues and cease treatment if needed. While it is illegal people will self-prescribe and self administer without a Dr to keep an eye on side-effects.

One of the interesting things about marijuana as medicine is that synthetic compounds that mimic it aren't as effective.