Normally I would agree with you, but the controls you are calling for are unrealistic. While I'm no expert in the field, this is clearly not the first study to suggest long-term cognitive decline in adolescent-onset users, and any evidence that can support previous findings, albeit very small, is important to consider.
I think the authors are correct in stating that there is a popular and naive belief among adolescents that cannabis is safe - especially in moderation - and the evidence to date does nothing but refute that. Hell, in this study they found a statistically significant IQ drop (over the course of 20 years) for adolescent-onset users that only smoked 14 days out of the year on average (the median, to be precise). Even if they do put forth incomplete or questionable conclusions, teens should be exposed to more of these findings and be allowed to think for themselves. They certainly should not have to endure the inane public service announcements that merely reinforce the belief that marijuana is not actually "that bad."