Not any more gross than a kid putting a cup that had a lid but had just picked his nose and then pushed the button to fill the cup. Germs don't stay in one spot. That whole machine is covered in germs. Specially since people will sneeze and cough all over it all day everyday.
Yes, my brain knows this. Guess you've never met a germaphobe before though. Those of us with OCD/germaphobia do not always think "rationally". Anything I can do to prevent any type of cross contamination helps me.