Us 'stupid' laymen know that if you take something without paying for it (when it has a sales price) - that's called stealing.
Actually, stealing is when you take something from somebody and that person no longer has it. If I take your car, then you no longer have that car. That's stealing. You can measure it by calculating the value of the car.
If I copy software you wrote, you still have the software. It was copyright infringement, a far different crime. True, the software has a sales price, like your car did. But will I just delete your software tomorrow, or produce 10 copies to give to my friends? Would I have bought it at the sales price if I couldn't copy it? You don't know. That's why the numbers on downloading are bullshit.
(BTW, I do plan on buying Doom 3.)