I have used UA twice in the last 6 weeks -both on transit via IAD and no price payed for onward domestic flight baggage (unless of course it is included in the price of the overall ticket).
From memory I think it states on the UA website that domestic flights used as connections from International flight do not have to pay for baggage - no?
well...according to the website it doesnt say so
Baggage fee exceptions
You are exempt from the above fees for checking your first two bags if you are:- traveling in United First® or United Business®;
- a Global ServicesSM, Mileage Plus 1K®, Premier Executive®, Premier® or Premier Associate® member;
- a Star Alliance® Gold or Silver member; or
- active military personnel and/or a dependant with ID and traveling on orders
it could be that the baggage fee was already included in your ticket