[Devil's Advocate mode = on]
It seems to be the 'digital' aspect of theft that has people confused.
For example:
If I take your car and don't give it back, I have permanently deprived you of it and is clearly theft. However, if I take your car, use it to make my own copy/replica and give it back without you ever realising, then how is it akin to theft in the normal/legal sense. No-one who downloads a film/book/mp3 is taking the original 'product' in the first place, so how can it really be 'theft'?
If you want to keep to a 'music' analogy, then:
Scenario 1) go to shop, steal CD, leave shop = THEFT as we all understand it
Scenario 2) go to shop, open CD case, put CD into laptop, rip CD to itunes, replace CD, leave shop = NO THEFT as nothing has been stolen
One argument is about whether the person who downloads/copies a file would have bought that file in the first place. If so, then it's a tangible loss to the manufacturer/publisher. If not, then they've lost nothing. No loss = no theft.
Another is about how much that loss is. If you've 'stolen' the file by downloading then the loss is only the amount of profit the manufacturer/publisher would have made on that product. That is, they can't claim they've lost a £10 sale when that £10 is made up of production costs, artist royalties, distribution costs, retailer margin, etc. The real loss to the manufacturer/publisher is only the net profit they would have made plus the royalties to the artist, which in some cases could be as little as 10p per album.
My personal argument for downloading some ebooks is that they've not been available from the publisher and have been scanned by a fan/pirate and posted online. I have a choice of either waiting for an indeterminate date when the publisher chooses to release the ebook version (at the same/higher price than the physical version, despite it not having the same overheads), or downloading it from the internet.
However, I don't think this excuse applies to any music/video/ebook where the digital version is readily available through 'legal' channels.
I'm not saying any of this is moral or legal, it's just my view of how 'downloading' is seen to more legal than 'proper' theft. In fact you could say 'theft by downloading' is more akin to receiving stolen property, as the person who downloads it has not ripped the CD in the first place - and is why all the prosecutions you've seen for 'copyright theft/infringement' are for people who upload/share these files.
[Devil's Advocate mode = off]