Most likely because IAD is a major station and can handle an extra car without a problem, while ORF is a much smaller station and if they got an iflux of extra cars, they might not have the room/insurance/carrying cost budget to handle them. The drop charge is essentially a way for them to offset the cost of caravaning to IAD to return the car to its home.

Could also be a corp vs franchise issue. Not sure on that, but a franchisee might impose the charge while a corp location does not.