From the Reason.com blog about:

"So is the ONDCP saying that marijuana causes teens to "engage in violent behavior, steal, use other drugs, and join gangs"? No. It is only strongly implying that. It calls early marijuana use "a warning sign for later gang involvement." It calls drug use a "risk factor," meaning that it's correlated with delinquency but does not necessarily contribute to it."

The whole thing seems kind of fishy.