Quoting myself from another thread:
As a business-to-business transaction (which this is), a MSC can contract or exclude anyone they want, at will, as far as I understand it. ADA laws would not apply, nor would any other employment regulation because this is not employment!
MSCs have a valid business right to choose someone who would best represent them for any assignment, even if you were an employee. Employment laws have all kinds of exceptions. Hooters won a well-known lawsuit years ago allowing them to hire only well endowed and good looking servers because it was the nature of their business. The only exception was that the court required them to have no limitations on other staff (cooks, bussers, etc.).
For all you know, your scheduler could be wheelchair bound, which would make them completely compliant in an employee based model....