It may not deprive the source from selling another copy, but not paying for your copy is stealing.
For the sake of argument, let's accept that definition and see where it leads us.
Well, why is stealing a bad thing in the first place? Is it because you get something for free? Surely not, because we all get things for free all the time. I can turn on the radio and listen to free music, then change stations when a commercial comes on. I can look at public murals that were funded by taxpayers who died before I was born. I can enjoy the benefits of those and countless other things without giving a dime to the people who created them.
I get upset when something is stolen from me, but is that because the thief has gotten something for free? No. If someone could "steal" a copy of my car, leaving the original car unharmed in my driveway, that wouldn't bother me at all. In fact, if the technology to do that existed, I believe it'd be a great leap forward for mankind.
We can also compare stealing to vandalism. If someone destroys my car, he doesn't gain anything for free, he only deprives me of the use of that property. Is destroying my car therefore not as bad as stealing it? It sure doesn't feel that way. In fact, stealing it seems marginally better, since it preserves overall utility (and there's a chance I'll get the car back).
So, I have to conclude that what makes stealing wrong is that the rightful owner is deprived of the stolen property. The benefit gained by the thief is only relevant to the extent that it comes at the owner's expense.
Now, what have we done by declaring that getting a free copy of something is "stealing"? We've created two categories of stealing: the old-fashioned kind where the owner is deprived of the stolen property, and the shiny new kind where he isn't. The first kind is wrong, since it maintains the quality that made stealing wrong in the first place. The second kind, however, is not - it's a benign, almost metaphorical type of "stealing", kind of like stealing second base. All we've accomplished with this new definition is to devalue the word.