NEARLY TWO years after Massachusetts voters approved medical marijuana, nine distribution centers are to open next year in places like Brookline, Lowell, Salem, and Quincy. The state licensing process for these dispensaries has been convoluted. And yet that path seems easy to navigate compared with the latest obstacle: the banking system. Because the drug remains illegal under federal law, marijuana-related businesses nationwide have struggled to find banks that will accept their deposits. This is creating a bizarre and dangerous cash management situation for businesses that Massachusetts and other states treat as legal. Federal lawmakers and regulators should acknowledge this and allow marijuana dispensaries to use the banking system safely.