On the grounds that nothing is taken... pure and simple.
The opportunity cost exists, as an economic concept that a potential sale is lost, however nothing is "taken".
Let me put it another way, if I made a copy of your car, you still have your car and I have an identical one. Your car has not been stolen. Hence Copywrite infringement isnt theft, theft is depriving someone of something. Its different.
Still has a potential loss, but it's not the same thing.
One would argue it's the equivalent of sneaking into a movie without buying a ticket. Receiving something for nothing sure, but not theft.
Honestly, you're giving the same argument all pirates give to pretend it's not stealing. You're really just trying to convince yourself that you're not a thief, that you are better than you really are.
Piracy falls into an interesting legal greyzone in many parts of the world, primarily in regards to whether or not someone is making money off of the pirated item.
In my own personal life I have to say that "piracy", the kind where no one is making money off of the aquiring of digital files, has led me to spend far more money to purchase legitimate copies of things I have "pirated". This is not the mindset of many people, who just want everything for free, but I really like to avoid "Mambo Number Five" syndrome where one song on an album is really catchy and then every other track is absolute shit and you only find out AFTER youve dropped 25$ on it (I can't recall an album this has personally happened to me with, but I know it has happened. The "mambo Number Five" example was the first time it was so bad that the album was unsalvageable, happened to a friend).
Further, without the era of "digital piracy" the fansubbing community wouldn't have been able to boom and I would never have been exposed to Kamen Rider, which has led me to spend significant dollar amounts on importing merchandise.
I've found that a few so called "legal artists" have in fact taken lyrics/music from others, calling it sampling so as not to pay royalties or just stealing it outright (Metallica, I'm looking at you!). Does "sampling" count as piracy? In my opinion it does but...there it is! They can do it because they're "sampling" or because they're Metallica/Black Eyed Peas/Andrew Lloyd Webber!
If I copy my CD music to mp3 and give a copy to my friends, does that count as piracy? But its my CD, I purchased it, its my property and I can use it as a coaster if I so feel like! And I have copied my music to mp3, since I have no intention of hauling two hundred plus CDs up and down.
Grabbing someone else's art and putting your name on it is WRONG! Wasn't there a big scandal a few years back, someone or other pretty much tracing parts of Jim Lee's Work from WildCATS and passing it off as theirs in one of the X-Men comics?
When it comes to software...I'm staying out of this. If the software manufacturers weren't such bast...Er...such jerks when it came to prices its likely there wouldn't be much piracy. Just look at Adobe. Greedy! CS4 has been around less than a year (a year?) and its already being supplanted by the new, supercalifragilistic CS5. And what does it have that's so great? A new hat?
Not only that, now the programs are broken up into suites for specialty needs of something, same with Windows 7.
Sure, you can have W7 for a low, low price but be ready to cope with a stunted version of the operating system. If you want more than you'll have to pay more. Taking the desktop customising features out of the basic version is a new low by Microsoft.
Lucky me I'm a pretty well off brat and can afford the nice stuff like Adobe and 3D Max or W7 Pro, but what of the people who are really talented and can't afford what they need? Damned Adobe and Microsoft!
And I don't purchase knock offs in any variety. Its annoying and the quality is rubbish.