Depending on where you are, a "regular" black coffee could also be an espresso. A drink which was made by adding water to it and removing the crema (the horror!) from it is the standard in some places. So, it's best to disambiguate it and I suppose that's why they force you to do that. To be honest, I never go to starbucks. So, I don't know if that is the case - I'm speculating from no real data.