Some restaurants serve certain products because of the contract with the company providing the fountain system service, which may have been chosen by the business owner due to contract cost, availability, or a myriad of other things. I assume you are asked as to not mislead the customer who specifically requested a certain beverage the restaurant cannot by contract provide/does not have on hand/does not supply. This is directed at Brookridge.
I haven't touched much soda since I was 14, but when I did, I usually preferred Pepsi, as my father was a Pepsi employee and probably ingrained that in my head. If I touch soda at all, it is usually clear or root beer soda. I'm usually drinking water or juice.