In my not so humble opinion, there are individuals who are willing to come up with all sort of studies to legalize the use of marijuana.
As more states legalize marijuana, adolescents' problems with pot decline
A survey of more than 216,000 adolescents from all 50 states indicates the number of teens with marijuana-related problems is declining. Similarly, the rates of marijuana use by young people are falling despite the fact more US states are legalizing or decriminalizing marijuana use and the number of adults using the drug has increased. Ref. Source 1j.
I fully believe these studies demonstrate why pot should be legalized. Pot is no more dangerous than alcohol (If you drink before your brain stops growing you are going to do permanent damage, if you smoke pot before then you will do permanent damage), but by decriminalizing it, there will be less 'street' vendors for 'kids' to get it from. This helps to decrease the number of 'kids' who will use marijuana. Just because more adults use it, is not a reason to criminalize it. Like I said I fully believe it is no more harmful than alcohol to society. Sure there are people who will become addicted and abuse it, but no more so than people become addicted and abuse alcohol. It is not the governments place to keep people from making 'bad' decisions when the decision is only 'bad' for a small subgroup. If it is bad for every single user, then obviously it is bad for society and should be regulated (Barbiturates, meth, etc), but otherwise(Like alcohol and pot) let people regulate themselves.
I don't know, studies can be biased and made to favor whomever is paying for the study. They can even make smoking and drinking sound good if there is enough money invested in the 'research'. There are so many herbs on the planet that can be used properly but the misuse of this particular plant leaves much to be desired.