I was once told by a rav that I am not allowed to buy a fountain soda in a not kosher place (like on a trip or something if I want to buy a drink) because we do not know what it is cleaned with. For all we know, they might be using their sponge that was used to clean ham to clean the fountain soda thing.
Don’t know if this is similar to a Mcdonalds type of thing but it reminds me of eachother since you can’t know for sure what they are cleaning the coffee pot with… could totally be wrong here though!!