No agency (or even U-Haul) will ever say, "Sure, Sir, please go use our vehicle and fill it with dirt! No problem at all!"

As for price, I suppose it does depend on his intended use. U-Haul charges $19.99/day + ..89/mile. Home Depot charges $19 for the first 75 minutes and then $69 per day. A National weekend rate of $14.99/day plus unlimited mileage makes a lot more sense as compared with those choices IMO, not to mention that you won't be stuck in a stripped out rattletrap.

I don't think the OP ever stated that DC-area trucks are not equipped with bed liners (unless that is how you read his comment "all the pickups i see are for prissy people to go to the downtown hotel.") Every full-size pickup (Dodge Ram, Nissan Titan, Ford F-150, Chevrolet Silverado) I have ever seen or rented in a rental fleet in the US in the past few years (that's 50+ rental locations nationwide) will have some sort of plastic bedliner. That is an option box that will always be ticked by the fleet managers because they don't want to return the trucks at the end of their service life with all sorts of gouges and damage to the bed.

Fact is that there is nothing in the contract or otherwise against the rules if OP chooses to use a rental agency pickup for hauling dirt, which is what his original question was. Common sense dictates not to return it looking like it went to the mud pits, but otherwise there is nothing wrong with doing what he is asking to do.