The economic costs of IP theft are a little murkier post-publication, but there’s still a fairly easily demonstrated opportunity loss. For example, I write a best-seller science-fiction novel. You write a sequel that takes my fantastic world building in another direction. You’ve done two things that damage my ability to make further money with sequels:
[ol]
[li]You’ve diluted my creation, the sci-fi universe and characters. If I stick to my plans, the continuity of my universe is fractured, which damages the overall property.[/li][li]Of the total money that the sci-fi universe has in it, to be exploited by writing, you’ve grabbed a share of it.[/li][/ol]Would Star Wars be the mega-property it is today if, in the time between Return Of The Jedi and The Phantom Menace, every two-bit film studio and publishing house had been releasing books and films set in the Star Wars universe?
Now here’s where the anti-copyright argument has a reasonable rebuttal: Multiple authors can create their own franchises of the sci-fi universe, and the market will decide who’s best; also, the overall market will be enlarged, enriching more authors overall, so my share is smaller, but more people get a share.
Here’s where the original argument for copyright comes in: by providing artists with a monopoly on their work for a set time, they’re encouraged to create it, knowing that it will enter the public domain after they’ve had a fair chance to exploit it. Without that chance, fewer would do so, especially smaller authors/musicians/etc. who’s work can be much more effectively exploited by large operations like recording studios and publishing houses.
That’s one angle. The other is the obvious one of, if you’re selling counterfeit copies of my book, then people are paying you for my work rather than me. When file-sharing comes into play, it’s a little murkier, since freely copying distributes the work more widely than it would be otherwise; nonetheless, it’s reasonable to assume that, of some of the people who got it for free, they would have paid for it absent the freely available copy. For those people, it’s money out of my pocket.
As for the Slashdot crowd, most of them understand that copyright is what give the GPL its strength, since the GPL is just a particular way of managing copyright. The reason they steal music and download films quite happily is because, like everyone else, there’s a fair number of people among them who rationalize it as a victimless crime or as Robin Hood redistribution of wealth.