Currently marijuana is such a problem in our society because it is not a real offense to use or possess it. Most offenders get community service and only get a maximum of a few weeks to a few months in jail/prison.
By making it a capitol offense, we would be able to make offenders do hard time for this and other "recreational drugs". Eventually repeat offenders would even be eligible for the death penalty.
If we keep allowing these people to get caught, get a slap on the wrist, and then be released, it will continue to spread the problem until it becomes an epidemic.