The Justice Department has decided not to fight 20 states and the District of Columbia over laws they have adopted, in contradiction to federal law, to decriminalize medicinal and recreational marijuana use.

Attorney General Eric Holder characterized the decision as establishing priorities for use of limited resources. Indeed, those resources should be used to fight criminal conduct rather than legal conduct in states that have adopted the new laws, usually by referendum.

If the DOJ had decided to take on the laws in court, it likely would have won because federal laws generally supersede state or local laws when they conflict.

The decision, while making sense as a practical matter, leaves unanswered some of the national-level problems that will arise from legal pot use in some states.

For example, what happens if a worker in another state is fired for a positive drug test after legally smoking pot in Colorado or Washington? And, while the DOJ will not fight in-state use, what happens when legally purchased pot is transported across state lines? What protections are in place to prevent legally produced marijuana, like legally produced cigarettes or alcoholic beverages, from becoming illegal black market contraband?

For now the DOJ decision spares the government the cost of litigation, but it doesn't resolve the many issues underlying the evolution of state-by-state pot laws. Eventually, they will have to be resolved at the federal level.