I don't understand how people can equate copyright infringement and stealing. Even if you're a proponent of current copyright laws, a quick glance at those very laws will inform you that they're quite different, and in fact copyright infringement can easily be committed accidentally and often commands much larger penalties than actual theft.

I think the argument is used so often because proponents of intellectual property protection realize that constructing an appeal to more axiomatic laws/rights breaks down very quickly. Traditional property rights are very easy to formally argue for by appealing to scarcity of resources. That obviously doesn't work for intellectual property.