It took less than a week for the Trump administration to kill the considerable buzz created Jan. 1 when California's broad marijuana legalization law took effect, creating the largest legal U.S. cannabis marketplace.

Then-U.S. Attorney General Jeff Sessions rescinded a policy shielding state-licensed medical marijuana operators from federal drug prosecutions.

The move sent a chill through the nascent legal industry. But ultimately it had little impact because federal prosecutors around the country showed little interest in going after legal operators.

Sessions, a staunch marijuana opponent, la...