Let's not get spun up and start losing track of what really happened here...
The Court did not uphold Oregon's assisted suicide law so much as it shot down the ham-fisted attempt to overturn it by the Justice Department. There is an enormous difference between the two.
It is still entirely possible for Congress to pass a federal statute, that is Constitutional, that outlaws assisted suicide. That would be an effective way to ban Doctor-asisted suicide, if that is the will of the Congress. Absent a Federal law that might have over-ruled Oregon law, the way that John Ashcroft chose to pursue overturning Oregon's Law was not the best way to attack the problem, and was doomed from the start. I do not question his motivation, just the method that was chosen.
This is a State's Rights issue, and that is how the Court appears to have ruled. It's early , and I have not read the ruling, but I've seen enough reports from different sources to be satisfied with that explanation.