The file-sharing/theft analogy is a fairly sound one. If I steal tangible property--say, a pack of gum--very few people would argue that my decision to do so was ethically defensible. Given that intangible property--say, a song--may be much more expensive to produce and arguably adds much more social value for having been created, why should we treat intangible property any differently?

Lets keep analogizing...

Like the manufacturer of the pack of gum, the song's creator has invested time and money into the development, production, and distribution of that product. And like the gum, the song has some intrinsic value (or else why would I take the time to download and listen to it?). Why should I be able to realize that value for my benefit without compensating the person responsible for its creation any more than I should be able to enjoy a delicious pack of bazooka joe without ponying up to the candy store?

One might argue that the ability to reproduce the digital download without imposing any cost on the creator of the intellectual property differentiates the intangible from the tangible. If my friend has 10 apples for sale at $1 a piece and I eat one while he isn't looking, he now only has 9 apples that he can sell, limiting his maximum gross revenue to $9. This feels wrong because the product is tangible & the loss quantifiable. On the other hand, if my other friend records a digital video that she is selling and I download it for free, she can still sell 10 more copies of the song and earn $10. However, had I paid for the download, she would have $11 rather than $10.

In each case, my unauthorized consumption left the person who had spent their own resources (both human and capital) with $1 less at the end of the day.

I might try and justify having downloaded the video for free by telling myself that my illicit consumption whet my appetite for her films, making me more likely to make purchases from her in the future. However, the same could be said for the juicy apple that I took from my other friend. Perhaps because I stole my first apple I will now come back and patronize his fruit stand regularly. Ultimately he will sell more fruit because of my indiscriminate apple theft, yet my original sin still feels unethical. As well it should.

When Curebit allegedly violated the intellectual property rights of 37 Signals, they suffered quite the backlash here on HN (http://news.ycombinator.com/item?id=3523024).

Of course, it is easy to distinguish between downloading someone's intellectual property for personal use and misrepresenting another's intellectual property as your own, especially for commercial purposes. Yet, at their core, both acts simply represent varying degrees of the same unethical behavior.

Like it or not, your gain, whether tangible or intangible, comes at the expense of the counter-party to the would-be transaction. Further, if enough people felt justified in their apple theft, it would be harder to find a decent fruit stand. Fruit vendors would close up shop or begin selling cheaper, lower-quality apples in order to minimize their losses to theft. Similarly, if enough people feel as though misappropriating intellectual property is acceptable, it is much more difficult for creators of such to continue to afford create.

The ability to monetize one's work, whether it be apple mongering or song writing, incentivizes and finances the production and distribution of higher quality apples and songs. Remove those incentives and both the artist and the entrepreneur will ultimately be unable to sustain their efforts in the long-run.