Legalized recreational marijuana in an increasing number of states will reduce unnecessary arrests for people in possession the plant, exhibitors and speakers at this week’s Marijuana Business Conference and Expo said.

“It’s time to roll back the war on marijuana and work toward fulfilling its potential,” Ethan Nadelmann, executive director of the Drug Policy Alliance, said during a keynote panel in front of about 500 marijuana business owners from across the country.

Founded in 2012, the annual Marijuana Business Conference and Expo returned to Las Vegas for its fifth year with more than 300 exhibitors and 7,500 participants. The four-day, industry-only conference, which started Tuesday at the Rio, targets owners of marijuana dispensaries, cultivation facilities, production facilities and testing labs.

Industry professionals on the showroom floor were all smiles today, a week after Nevada, California, Massachusetts and Maine passed ballot measures allowing the recreational use of marijuana. Arkansas, Florida,...