I mean, are you taking your definition of "theft" from the law? Or from your own internal set of ethics for right and wrong? Is it theft if no one is deprived of anything, because bits copy, and because you'd never trade dollars for the privilege of maintaining an exploitative relationship with a company but that is all they've made available?
If you're hung up on whether the legal system thinks it's theft - I dunno what to tell ya, it obviously does.
Edit: uh, maybe you're literally asking for how the logic in that statement works, which I read as just "if it can't be owned, how can it be stolen?"
Oh, yeah that makes sense too. Bad premise all around I guess.