It's a well-known concept that federal law preempts state law.
Apparently, as per LA Times Op-Ed The case against Colorado's pot law, Nebraska and Oklahoma are suing Colorado to invalidate Colorado's recreational marijuana law, since the border cities of some border states have already collected far too many visitors from Colorado in their lock-up facilities. (Some towns/sheriffs even go as far as to claim that Colorado must share Colorado's revenues to allow the neighbouring states to prosecute such Colorado visitors!)
The Op-Ed states that the situation is rather clear -- however outdated or incorrect it may be, federal Controlled Substances Act does preempt Colorado Amendment 64 (as well as the similar law of many other states where cannabis is not illegal), even if the previous attorney general, Eric H. Holder Jr., didn't feel like prosecuting anyone (but the office now has a new AG).
But why is the Supreme Court asking the Attorney General for a brief? Isn't the law clear enough as it is?
Can Colorado use the well-known evidence that the federal law is simply outdated and was never really correct to start with, contradicting medical advice since the start?
Likewise, if some states do feel that prosecuting their own citizens who visit Colorado is a worthy pastime for their border cities, why don't they simply put their money where their mouth is, by raising the taxes on their own residents, to make sure their border cities have adequate lock-up facilities?
Is the Supreme Court at all allowed to do something like the Jury Nullification here, to declare that the justice would not be served if the outdated federal law was to preempt the realities of the many states?