“Because of regulations that bar banks from doing business with drug traffickers, many financial institutions refuse to open accounts for dispensaries or give them small-business loans. The same goes for major credit-card companies like Visa and MasterCard. Merchant service providers—the middlemen between retailers and credit-card companies who process customers’ payments—are also reluctant to run afoul of the federal government, so most won’t accept payments from dispensaries. These restrictions force dispensaries to operate as cash-only businesses. Without a bank account, dispensary owners can’t deposit their money in a secure place or write checks to their landlords.”
http://prospect.org/article/life-takes-visa%E2%80%94except-if-you-want-buy-pot