t's now legal to use marijuana, medically or recreationally, in 28 U.S. states. But many cannabis businesses still can't access the banking system — and that has repercussions far beyond where dispensaries store their cash.

Marijuana remains illegal to buy, sell or possess under federal law. And because most banks are insured by the Federal Deposit Insurance Corporation (FDIC), they must comply with federal law. A 2014 regulation allowed banks to accept cannabis revenue, but the majority still refuse to do so — and if they learn that a client is depositing theserevenues in an existing account, they often close that account. Entrepreneurs seeking startup money need to turn to private backers.

But now, cities and states are taxing the industry, often collecting millions of dollars in revenue. As the Trump administration enters the White House — with strident anti-marijuana Sen. Jeff Sessions, R-Ala., nominated to be its Attorney General — the industry, and those who benefit from the money...