Butter?

I had a baked potato at lunchtime.

I have noticed that both of the food outlets near my work which sell baked potatoes make the same mistake.

They always ask "do you want butter on it?" to which I always reply "yes".

They then, invariably, put margarine on the potato. Not butter.

Are there people out there who really don't know the difference between the two? Do thay all work in baked potato shops? Why, oh, why?