Canadian cannabis companies have it easy. Since Oct. 17, they’ve been able to sell pot to adults under a single national law. The big U.S. stock exchanges are happy to list Canadian producers like Canopy Growth (ticker: CGC) and Tilray (TLRY).

It’s harder for America’s cannabis businesses. U.S. federal law still considers marijuana an illegal drug in the same class as heroin. Any financial service provided to a cannabis business may violate federal law. So bankers in the Federal Reserve System, not to mention the New York...