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.
Making marijuana available for medical purposes is basically decriminalization. I drew the distinction as I've encountered people who think that states are legalizing it which is an entirely different thing.
Practically speaking decriminalization is making pot available to a much wider audience at a lower cost as (at least in Denver) there's no practical limit to how much people can buy. One person with a medical marijuana card can (and does) supply a fair number of people without.
