Amazing, someone as OCD as me! ;D
I used to wonder the same thing, but I honestly put it down to paranoia! The reason being some of the people that did it were loyal customers (and practically friends), that loved our coffee. I never asked why they did this, just assumed they did it growing up and never broke the habit?
Id say that if they didnt like the coffee, they wouldnt drink it. Or just half of it.