I like how the hot dog stand has signs for soft drinks from three different soft drink companies. Now days restaurants almost always have a deal with one company and only serve those brands. It would be nice to be able to get a Coke or a Pepsi.
There are places that sell both. Arco mini marts have drink machines that dispense both, for example.
Mini marts are different from food vendors... they don't have similar arrangements. If anything only for promotions, not exclusivity like a restaurant chain would agree to.
I think they provide the machines as an incentive, and in turn it can only dispense their brands.