Here's my take: Having large quantities of cash is a security problem for the marijuana businesses. If retailers start incentivizing their patrons to load up a "green card" at certain dispensaries acting as the cash funnels, so to speak, then the cash is centralized, making the logistics of storing/moving/ placement
easier.
Plus, retailers who may have been interested in sales of other things besides marijuana can accept payment for those things via the traditional merchant services processors, and accept payment for marijuana products through the stored value cards. Perhaps they (FastFunds and company) believe that more retailers would be interested in offering kmarijuana products if they didn't have to deal directly with the cash and its associated headaches.