Skipping to the end of the article, we find SCPD Lt. Saunders claiming that determination of whether a person is a legitimate marijuana patient is judged on a "case by case method".

Wouldn't a letter from their doctor be the clear and convincing evidence the police need to make their decision?

Since homeless people and youth are constantly targeted by the SCPD for marijuana enforcement, maybe this issue isn't as "frequently blurry" as Saunders suggests, but has more to do with the economic status of the user.