The U.S. Supreme Court on Monday refused to consider a lawsuit from Nebraska and Oklahoma against Colorado's regulated sales of marijuana for recreational use, removing a major threat to the multibillion-dollar state-legal cannabis industry.

Possession of marijuana for any reason outside limited research remains a federal crime, and marijuana reform advocates feared an unfavorable ruling would take a wrecking ball to laws in four states that allow recreational pot sales and medical marijuana laws in many others.

Colorado's neighbors argued the 2014 opening of recreational marijuana stores selling small amounts to anyone 21 and older caused an increase in cross-border criminality, and that Colorado's regulation of pot stores was illegal under federal law.

The Obama administration has allowed states broad leeway to regulate marijuana sales, even though federal prohibition remains intact, and urged the court in December to dismiss the states' lawsuit. A future administration likely could end executive branch permissiveness.

The six-justice majority that refused to hear the case did not explain themselves, but two of the court's more conservative justices said the case should have been allowed to continue. If it had, the Supreme Court – which has original jurisdiction over disputes among states – would have appointed a special master to develop recommendations, which the full court would then consider.

"Whatever the merit of the plaintiff States' claims, we should let this complaint proceed further rather than denying leave without so much as a word of explanation," Justice Clarence Thomas wrote in a dissent joined by Justice Samuel Alito.

Marijuana reform advocates rejoiced in the defeat of an existential threat.

"There's no question about it: This is good news for legalization supporters," says Tom Angell, chairman of the group Marijuana Majority. "This case, if it went forward and the court ruled the wrong way, had the potential to roll back many of the gains ... and the notion of the Supreme Court standing in the way could have cast a dark shadow on the marijuana ballot measures voters will consider this November."

Don Duncan, California director of Americans for Safe Access, a medical marijuana advocacy group, says the decision continues "a long history of courts giving states latitude" over marijuana policy.

Duncan says the decision is further proof that courts will not overturn state-level marijuana reform laws.

The Supreme Court previously passed over an opportunity to kill state medical marijuana laws in its 2005 Gonzales v. Raich decision, which upheld the ability of Congress to make marijuana possession a federal crime, even in states that allow its use. And though an opinion was not issued, the court's non-action in 2014 forced Arizona police to return medical marijuana to a California resident.

Though the Nebraska and Oklahoma lawsuit did not challenge medical marijuana laws, which allow for regulated cannabis markets in 23 states and the nation's capital, those laws likely would have been affected had the Supreme Court struck down Colorado's recreational marijuana law, Duncan says.

"The same principle applies," he says. "It was a potentially disastrous case for patients if it went the wrong way."

Colorado's Republican attorney general, Cynthia Coffman, defended the state against the legal challenge. She told U.S. News last year she personally opposes pot legalization but saw it as her job to defend state laws, and in a statement Monday indicated continued discomfort with legalization.

"I continue to believe this lawsuit was not the way to properly address the challenges posed by legalized marijuana, but the problems are not going away, period," she said. "Although we've had victories in several federal lawsuits over the last month, the legal question surrounding Amendment 64 still requires stronger leadership from Washington."

In January, a federal judge in Colorado dismissed a lawsuit dealing with marijuana legalization brought by the anti-drug group Safe Streets. Another federal judge in February dismissed a lawsuit filed by a group of sheriffs, finding no private right of action for the officials to enforce the federal Controlled Substances Act or international anti-drug treaties.

Oklahoma's GOP attorney general, Scott Pruitt, said in a statement he wished the Supreme Court had used its authority “to address the unwillingness of President Obama’s Justice Department to do its job, namely, enforce the law against the illegal flow of marijuana into ... states across the country.”

“Colorado marijuana continues to flow into Oklahoma, in direct violation of federal and state law," Pruitt said. "Colorado should do the right thing and stop refusing to take reasonable steps to prevent the flow of marijuana outside of its border. And the Obama administration should do its job under the Constitution and enforce the Controlled Substances Act.”

Nebraska Attorney General Doug Peterson, also a Republican, chose to downplay the Supreme Court action, describing the development as unimportant.

"Today, the Supreme Court has not held that Colorado's unconstitutional facilitation of marijuana industrialization is legal," Peterson said, "and the court's decision does not bar additional challenges to Colorado's scheme in federal district court."

Kevin Sabet, leader of the national anti-legalization group Smart Approaches to Marijuana (SAM), says the decision is "obviously a disappointment, but we never put too much stock in these court cases anyway."

Jeffrey Zinsmeister, SAM's executive vice president, tells U.S. News in a statement "the lack of any explanation in the order (which is common) means that trying to divine the court's reasoning is armchair speculation at best."

"Overall, much ado about nothing," he says. "The court's unsigned opinion just means that the case has to be filed in federal district court like most federal lawsuits."