The operative difference is that stealing actually deprives someone of something, necessarily.
When it comes to copyright infringement, you're really violating the rules put in place that grant copyright holders a limited monopoly over their creations (its supposed to be limited, anyways). It is true that in some cases, those violations will deprive them of actual sales... in many others though, they will experience no loss at all.
And its always worth keeping in mind, that the terms of copyrights have increasingly, over the years, been tilted in favor of the copyright holders - far, far, beyond their original design - through intense lobbying by organizations like Disney, the RIAA, and the MPAA.
In short, I think the morality is a lot fuzzier when it comes to copyright infringement - than it is with actual theft.









