I'm not much in the mood for this after the 1,000th time, but copyright infringement isn't theft. Saying otherwise is being willfully ignorant at best, or knowingly using incorrect terminology to influence opinion at worst.
Saying that digital copyright infringement is not theft is a bit like saying that someone who runs off with a bike his friend borrowed from a third party did not steal said bike.
That said, the legal analysis probably differs depending on which side of the infringement chain one is on.
For example if you are distributing copyrighted material illegally, you are infringing copyright through the distributive act, presumably because the content owner has not granted you the right to distribute the content (precisely because that is how content owner's make money). That said, it would be relatively hard to establish that illegal distribution is theft. The argument would have to rely on a position that the property that was stolen by the distributor was the copyright owner's right to distribute the copyrighted material. That would be a difficult position to establish except in rare circumstances, as theft (e.g., larceny) would likely require that the copyright owner establish that the distributor intended to permanently deprive the copyright owner of the right to distribute.
On the other hand, if you are downloading copyrighted material, an argument could be made that you are stealing the material being downloaded, as well as infringing the copyright on the material. Although it varies by state, larceny is often defined as (1) the unlawful taking and carrying away of someone else's property, (2) without the consent of the owner and (3) with the intent to deprive the owner of the property permanently.
In the case of digital content, a person who lawfully obtains a copy of copyrighted material ("licensee") generally has a license that gives them right to possess and use the material within the bounds of the license (which usually excludes the right to distribute). To be clear, the licensee
does not own the licensed copy of the digital content,
the copyright owner does. Don't like that fact? Don't buy a limited license to copyrighted content.
With the foregoing in mind, a person who downloads ("downloader" ) a copy of that content from the licensee is not obtaining the property of the licensee, because the licensee does not have rights beyond those extending to personal possession and use of the material. Rather, the downloader is acquiring the copyright owner's content without permission. That is, the downloader is unlawfully taking and carrying away the copyrighted digital content, without the consent of the copyright owner and with the intent to deprive the copyright owner of the downloaded copyrighted material ("in this case, a specific copy thereof) permanently.
Sorry man, its pretty hard to convincingly argue that, in the context of illegally downloaded digital content, that copyright infringement is not a form of theft. The fact that one might think they own content when they do not does not change the legal analysis. Ignorance of the law is no defense, as they say.
Of course, the above is not legal advice and is not intended in any way to be an exhaustive argument or analysis with respect to this topic. I'm not your lawyer, and you rely on anything posted here at your own risk.