Digital goods are fundamentally non-excludable and non-rivalrous (sometimes anti-rivalrous, as with software). Thus the semantics of "theft" do not apply. At worst it would be free-riding, which has different implications.
The idea that theft has to include rivalrous good is a strawman. You are just taking the difference between theft you don't like and pirating and then saying that difference is hugely important.
> Whoever transports, transmits, or transfers in interstate or foreign commerce any goods, wares, merchandise, securities or money, of the value of $5,000 or more, knowing the same to have been stolen, converted or taken by fraud; or ....
So the people trying to get through the not stealing it is copying due to a technical definition of theft are wrong. Stealing is having a copy that you didn't get permission to take.
My other ethics brain is wondering how NOT taking property but a copy seems to also allow sneaking into a movie theater, concert or sporting event to watch something. You are neither taking anything or even copying you are just watching. Is this also not theft?
In which case properly delineating what "asking for permission" is becomes increasingly intractable. I have all sorts of images in my thumbnail cache that my browser has downloaded which nominally I do not have permission to keep, but are unavoidable artifacts of using the web via a graphical browser. Indeed, the lines of what is and is not implicit consent are very blurred. Not to mention that merely not being supposed to have something under a set of terms says nothing of the legitimacy of said terms.
A lot of companies have provisions in their EULAs to allow for copies stored in RAM or by the browsers, e.g. http://storedvalue.com/en-US/terms-and-conditions and I believe streaming services are doing that as well since going to the monitor will require making copies and transforms of the data from the network. It's all pretty funny until someone gets sued.
Yes this is the cause of many crazy Internet stories of Instagram or FaceBook now own your photos.
Photos on a public web page are implicitly giving you permission to view the photo and the technology need you to have a local copy for viewing. That does not mean you can grab the photos from say National Geographic and re bundle them put them on you website surrounded by adds. They are not giving you permission to do anything but view the pictures.