My guess is that they're allowing the leniency to the few states that blaze trails.....CA, CO, WA for the sake of future argument. They spend many tax dollars on policing those drugs and even jailing people over sales and possession. Overcrowded jails are a big problem in CA, one of the reasons they passed it out there...costs the state millions a year just in staffing, food, and other expenses they never get back....on the flipped side, the regulation of the industry is generating huge tax dollars and giving the drug trade a black eye in those states.
My assumption is that more and more states will jump on board as they see potential tax profits and as we move further from Christian values in this country. I suppose with pot and alcohol being readily available, maybe people will stop getting stuck on painkillers and meth, but do it indoors off the streets.
What concerns me the most is that when things are more available, the population of people who wouldn't have tried it or tried getting it will simply walk in off the street and then not know how it will affect them. At least, from what I read in Colorado, the prices are so much higher than the price of street drugs it will keep usage low for most people.