The three dissenters noted that such a conclusion is contrary to the law's intent (to protect the public from impaired drivers) and inconsistent with state and federal criteria for Schedule I substances (which are supposed to be psychoactive chemicals or precursors to them). They also argued that the ruling results in an unconstitutionally vague law that invites arbitrary and discriminatory enforcement.
Given variations in metabolism and laboratory standards, marijuana smokers can never be sure whether they're legally permitted to drive in Michigan. The statute as interpreted by the Michigan Supreme Court therefore does not give people enough information to know when they are violating it -- a basic requirement of due process and the rule of law.
Treating unimpaired drivers as if they were intoxicated is fundamentally unfair, and treating a drug metabolite with no pharmacological action like the drug itself makes no sense if the goal is preventing accidents. But the drug warriors who see Michigan as a model for the nation have other goals in mind.
Proponents of "zero tolerance" laws such as drug testing consultant J. Michael Walsh and former federal drug czar Robert DuPont see them as a way of deterring drug use and forcing users into "treatment." If the point is to make the penalties for smoking marijuana more severe, let's have a debate about that, instead of pretending the issue is traffic safety.