This is standard and sometimes very frustrating. I was once booked WAS to CYN on the Carolinian and traveling with two friends who were booked WAS to RGH. The attendant demanded we sit in separate cars, because not all cars open at CYN and apparently having me move cars after RGH would be too confusing. (I finally won that fight by simply saying, fine, I'll get off ten minutes earlier at RGH.)