The criteria for what makes a substance banned is totally subjective. There's a ton of stuff on sale at GNC that could get you busted. Marijuana only gets busted because it's not yet legal in certain states. It's not physiologically addictive*, isn't particularly performance enhancing (at least, physically), and does not pose any documented harm to its users. Honestly, my image of a fighter only takes a turn if I see steroids or something of the like, not weeds. I heard Diaz's hearing is the 17th of February and since it's his 3rd offense, I wouldn't be surprised if they kicked him out of the UFC. It would be a travesty.
Side note: I learned in Pharmacology class that the hallmark of physiological addiction as it relates to the brain is 'dopamine stimulation in the nucleus accumbens,' which is the almond sized 'reward center' of your brain. Most drugs stimulate this, including alcohol, coke, heroin, vicodin, painkillers, and a ton of others. The reason Sex Addicts exist is because brain-wave activity taken from people experiencing an orgasm is strikingly similar to brain-wave activity of someone who's high on heroin, albeit for roughly 3 seconds compared to heroin or coke, which would last 5-10 minutes with a lot of residual effects afterwards. 3 seconds is a natural process with how we've evolved. 5-10 minutes is unnatural. It's like if you floored the gas on your car pedal for 100-200 times longer than you usually would. It could cause your brain to fuse out. People get used to this feeling and want to chase it as much as possible. It is the maximum stimulation of the reward center of your brain. This is why ecstasy can be dangerous if taken a good amount in a short amount of time, because the brain can effectively relapse due to the overstimulation of dopamine/serotonin receptors and what happens is these receptors literally Prune Up, so you can't experience 'happy' feelings as much as you used to. Another way of putting it is that your brain becomes 'desensitized' to experiencing rewards. This is why food might not taste as good, a joke might not seem as funny, or a chick might not seem as hot. It's why you experience a 'crash' if you ever take a drug of abuse. Marijuana has been PROVEN to not be physiologically addictive as it pertains to what PRECISELY defines a drug with the potential of abuse. So anybody that categorizes Marijuana as a drug of abuse (as do several drug testing organizations) is ignorant of it's addictive potential. Much like what Joe Rogan said. "Are you gonna outlaw cheeseburgers cuz there are fat people that eat a lotta cheeseburgers?"