Maybe because everyone knows that coffee is coffee and that even though they sell other products the main purpose of the store is selling “coffee”. So one should dan l’kaf zchus and assume that anyone that stops in is buying just that “coffee” or a similar kosher beverage. AND in this day and age if you see a YID coming out of McDonalds it is also normal to assume that he stopped to use the “clean” bathroom and buy a soda and not that he stopped to eat treif.
It is very interesting how no one will stop you at a McDonald’s, 7 eleven, or Dunkin Donuts from using the bathroom but in many other stores including kosher stores they will tell you that the bathrooms are for “patrons only”. We can all understand that, but when you are on the road or in a dire emergency you really don’t want to hear that you just really need patience and understanding.
So by the same token, we must teach ourselves and others to Dan L’kaf Zchus and assume that if you see a Yid come out of one of these establishments or one similar s/he needed to use the facilities or stopped to buy a coffee or soda.