Sadly, in so many other areas of our society, repeated use of drugs would be considered an addiction and, possibly, even a disability. The focus would be treatment and rehabilitation. Too bad that avenue was not available to Mills.
I wish that were more true. Even in mental health care, drug abuse is often poorly understood and causes discrimination, and access to real evidence-based treatment can be scarce. Treatment centers may be heavily commercialized, in some cases outright fraudulent, in other cases pinned to a religious organization offering a service (grateful for that) but may push faith and not follow any evidence base for successful addiction treatment. Insurance usually provides disparate benefits for mental health and even worse for substance abuse. Addiction is an extremely difficult disease to treat to start with, whose very definition is essentially that someone is unable to control their behavior despite having clear negative consequences. Trying to force somebody to be accountable for something they don't have control over is plain dumb. Addictive substances are addictive because they hijack the circuitry in the brain that regulates self-control. No matter how much someone's cerebral cortex learns that the behavior is bad, they're always going to be battling the primitive brain that is permanently wired to compel otherwise. Cannabis is an addictive substance. It influences that circuitry in the brain. It is true both that cannabis addiction is less likely compared to other substances and has much less risk of adverse consequence, but some people who use it are going to become addicted, and when that happens it needs to be recognized as an illness. There are a bunch of people out there that can drink a 2 liter of coke a day and have a donut for breakfast and never get diabetes, but those who do get diabetes don't need shame for their bad eating habits -- they need treatment.
I don't know Dedrick and couldn't say whether he has an addiction, but it is clear that he knew the consequences of using and when he'd be tested for them and still didn't control his use. We know other players smoke pot, yet they have been able to keep themselves clear when it counted.
The policy is bad, though. It makes no sense to punish someone for having an illness. If the athletic department, the Institute, the ACC, the NCAA, etc. care about players using marijuana, then the policies should be centered around helping them if they detect a problem. Contingency management could be part of that and does enhance success, but that's only useful if treatment comes first. Otherwise, if they aren't there to help anyone, they should stay out of it unless the behavior is somehow damaging to the team, the school, the conference, or college athletics overall.