I don't think it is. When some one steals something, the important part is that someone unjustly lost their property. When you file share no one loses their property so I don't think its stealing.
Sure someone loses potential profit but does that constitute stealing? No.
(This is a far-fetched example but I believe it illustrates my point) Lets say for instance I go out and watch a movie, then when I get back I describe the movie to my friends scene by scene, including the ending. Now since they know the story and the ending they don't go see the movie they would otherwise might have paid to see. Is that stealing? No. But in effect I 'shared' the movie with them. Same thing.
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