I bet they would count McDonalds as employed. They probably require the school to fill out a form, and the school probably counts anything they can as employment. Does US News make a distinction as to "private practice" or not? If so, there's probably an "other" category so McDonalds employees don't get counted as private practice. I know that BU is keen to tell you the median private practice instead of the average. With a system like that, a couple McDonalds managers wouldn't have any effect on the number (assuming there were a couple big firm lawyers matching them at the other end of the scale).
Of course this is mostly just a theory of mine. I don't, for the record, expect to see any of my classmates working at McDonalds... because I never go to McDonalds.