Some in the marijuana-reform community also criticized the memo, noting it did not represent a fundamental change in the law, which would require the approval of Congress.
"It's like, 'We're going to be tolerant of this as long as we feel like it,'" argued Seattle marijuana defense attorney Douglas Hiatt. "Is a new administration just going to come in and shut it down?"
But others pointed to language in the memo they found remarkable coming from the Justice Department: an acknowledgement that a well-designed regulatory system could actually help achieve federal law enforcement goals.
"Indeed, a robust system may affirmatively address those priorities by, for example, implementing effective measures to prevent diversion of marijuana outside of the regulatory system and to other states, prohibiting access to marijuana by minors, and replacing an illicit marijuana trade that funds criminal enterprises with a tightly regulated market in which revenues are tracked and accounted for," Cole wrote.
A Pew Research Center poll in March found that 60 percent of Americans think the federal government shouldn't enforce federal marijuana laws in states where its use has been approved. Younger people, who tend to vote more Democratic, are especially prone to that view.
But opponents are worried these moves will lead to more use by young people. Colorado and Washington were states that helped re-elect Obama.
Kevin Sabet, the director of Project Smart Approaches to Marijuana, an anti-legalization group, predicted the new Justice Department policy will accelerate a national discussion about legalization because people will see its harms — including more drugged driving and higher high school dropout rates.
Kristi Kelly, a co-founder of three medical marijuana shops near Denver, said the Justice Department's action is a step in the right direction.
"We've been operating in a gray area for a long time. We're looking for some sort of concrete assurances that this industry is viable," she said.
Yost reported from Washington, D.C. Associated Press writers Alicia Caldwell in Washington, D.C., Rachel La Corte in Olympia, Wash., and Kristen Wyatt in Denver contributed to this report.