Last month, hundreds of visitors mingled outside three conferences being held side by side in New York City’s massive Javits Center. Sharing lines for lattes with their neighbors from the Salesforce and International Franchise Expo meetings down the hall, attendees of the Cannabis World Conference & Business Exposition could only have been picked out of the business-casual crowd by a careful observer thanks to one detail: the green accents on their name badges.

Inside the International Cannabis Association-hosted conference (just one of many such marijuana-themed trade events of late), dozens of exhibitors advertised services and products dedicated to the business of marijuana; namely, how best to develop, manage and profit from “Green Rush” enterprise that’s brought investment billions to several Western states and has been creeping nationwide.

One exhibitor, representing all-things-analytical instrumentation for GenTech, offered passersby colorful marshmallow squares; treats which, he assured visitors, did not contain cannabis. He frowned, saying, “Everyone keeps asking...