The state of Montana passed a law that made medicinal marijuana legal under state law, but it remained a federal crime subject to being charged and convicted at the governments discretion. All the law in Montana stated was that Montana agreed not to prosecute for those crimes, but the federal government made no such commitment.
The fact is that Chris Williams broke the law and was charged, and lawfully convicted, so there should be no special treatment and no pardon. He should spend the next 80 years behind bars for his crimes.