are you doing a mileage run??? if so, why are you checking baggages???
its legal for them to check your bags with your flight routing like BRU-IAD-BOS-IAD-DXB, but having them check BRU-IAD-DXB might be illegal since you will not be on the IAD-DXB flight.
also, i assume you are booked like this: BRU-IAD-BOS and BOS-IAD-DXB
if you do this, although you can skip the IAD-BOS leg, but you can not miss the BOS-IAD leg without having the reservation canceled.
good luck.