Justice Anthony Kennedy, writing the majority opinion said that the federal government does have the authority to go after drug dealers and pass rules for health and safety, but power was not intended to extend to the Oregon law. Kennedy said the "authority claimed by the attorney general is both beyond his expertise and incongruous with the statutory purposes and design."The majority opinion included retiring Justice Sandra Day O'Connor and Justices John Paul Stevens, David Souter, and Ruth Bader Ginsburg, and Stephen Breyer.
Roberts and Justices Clarence Thomas and Antonin Scalia dissented. Scalia commented that the court's ruling "is perhaps driven by a feeling that the subject of assisted suicide is none of the federal government's business. It is easy to sympathize with that position."
More on the court's reasoning as soon as the bench opinion becomes available.