Although I disagree with the DOJ’s interpretation of the law, it isn’t entirely unreasonable – as evidenced by the fact that several federal district court judges agreed with the DOJ position (the dispensaries were appellants in the appeal to the 9th Circuit).
I don’t think the fact that district judges agree with an interpretation makes it reasonable any more than the fact that the DOJ initially adopted it makes it reasonable.
What makes something reasonable is not a function of who espouses it. Naturally, sometimes we use rules of thumb to assess the reasonableness of positions too complex for us to realistically evaluate. But this is not such a case, since we can look at the statutory language with our own eyes, and the legal test involves assigning that text its plain meaning.
The statute says that the funds cannot be used to “to prevent such States from implementing their own State laws that authorize the use, distribution, possession, or cultivation of medical marijuana.” The question is whether arresting the individuals licensed by the states to cultivate medical marijuana prevents them from implementing their laws. In any other context except federal law enforcement–to which the federal judiciary is extremely deferential, in part because many (most?) of them are former federal prosecutors–the answer would be so obvious that no one would disagree.
The fact that district judges agreed is absolutely relevant in a reasonableness discussion, because it suggests that reasonable minds differed on the interpretation of the statute. For example, in lawsuits where the government is a losing party, the opposing side can sue for reasonable attorney fees so long as the government’s litigation position was substantially justified (i.e., reasonable). Under Ninth Circuit caselaw, the fact that other judges (even in the lower courts) agreed with the government’s position is a factor that can be cited in support of reasonableness.
Again, I don’t necessarily agree with the DOJ’s interpretation in this instance, but it is not entirely an unreasonable one.