Fuel is fuel is fuel. The tech regs, at least from what I can remember specify a pretty clear "dry weight" of car and driver at 600 kg. No car would run without fuel in it, obviously, but fuel is not part of the racing weight requirements. The car must weigh at least this much during the entire grand prix. Obviously if you use fuel as weight ballast, your intention is to burn it off because gasoline doesn't make good ballast, being a liquid and all. BAR got caught with their pants down and are going to try and weasel their way out of it. I would hate to see them excluded, but I hate cheaters even more. "Tolerances" and grey areas are one thing, but lying to an FIA official by arguing that you didn't understand what " drain the fuel from the car" means and then arguing that it is an integral part of your engine is just dumb. Air is a vital component to combustion too but I don't see anybody trying to argue the "integral" nature of a turbo charger.
No matter how much they test, the F2005 weighs 600 kg when it lines up on the grid, that's the difference.
Ferrari's testing is not something that is regulated by the FIA, that was simply a good faith gentleman's agreement that to my understanding Ferrari was not initially invited to sign. Good faith and a dollar will get you a cup of coffee. Ferrari should have probably played ball on the testing agreement, but it is certainly not a rule of law.