What you're saying makes sense, but marijuana is a ridiculously finicky drug when it comes to how it effects people.
It's not like paracetamol where for probably almost everyone it's a case of take 2 pills and bamwhammo, you're headache is gone, and perhaps take one more but it wont do jack shit
With pot (I recall you said you don't use it, right?), it's almost an art form trying to get that right amount in your system.
The difference between 2 and 3 hits of a bong can be quite staggering for some people and ultimately there is no doctor in the world that could suggest a well defined dosage, because it truly does vary a hell of a lot from person to person, so I don't think it's so unreasonable that the user figures out the dosage themselves depending on their ailment/s.
You could argue the same thing for analgesics, but unlike Paracetamol, NSAIDs and opioids, there is no risk of accidental overdose trying to figure out a good dosage level.
At worst, you'll just pass out and wake up many hours later wondering what the hell happened.