I believe you misunderstand what "decriminalization" is. Marijuana being "decriminalized" simply means it is not a felony to possess it in amounts intended for private use. This is the case in a number of states including Ohio where I grew up. In Ohio they'd joke about it being a "50$ fine"... I don't know if that's the actual fine but there is no fail time and simply some fine for marijuana possession in small quantities.

Also, in many of these states you'll find that enforcement of marijuana laws are fairly lax in general. A friend was pulled over with marijuana in their car and the judge changed the charge to "public disturbance" or some such so that they wouldn't lose their student loan eligibility and other things.