There's no good reason for ANY criminality to be associated with casual marijuana use. Why should it even be a ticketable offense? Don't just decriminalize it, legalize it!
If people who wanted to use marijuana were able to grow their own, they wouldn't need to buy it from unknown sources. Sending people to jail for marijuana costs taxpayers a fortune. We don't send people to jail for buying a case of beer or a bottle of whiskey so why should we send them to jail or fine them for marijuana possession?