How did the CFIB get a discount?

I am AMAZED that the CFIB could negotiate a discount for their members. The reason is that almost all their members already have no choice but to accept MasterCard. (Only a powerful company like Costco is in a position to refuse certain cards). The CFIB could not credibly threaten to have its members favor Visa. The customer decides. I suspect MasterCard was just ready to give a discount based on lower costs and obese profits.