Theft \Theft\, n. [OE. thefte, AS. [thorn]i['e]f[eth]e, [thorn][=y]f[eth]e, [thorn]e['o]f[eth]e. See Thief.] 1. (Law) The act of stealing; specifically, the felonious taking and removing of personal property, with an intent to deprive the rightful owner of the same; larceny.
Note: To constitute theft there must be a taking without the owner's consent, and it must be unlawful or felonious; every part of the property stolen must be removed, however slightly, from its former position; and it must be, at least momentarily, in the complete possession of the thief. See Larceny, and the Note under Robbery.

Copyright infringement is not theft. That's why we use a different phrase. To pick an example, completely at random, scanning a comic book, and placing it on the Internet is not theft. How can it be? The owner still has the comic. He is still breaking the law, as copyright rules prohibit such distribution without the agreement of the author, but it's not theft.

Incidentally, I find it amusing that the comic in question has such a twisted copyright history. If Carl Henderson and/or the Kimota! book are to be believed, then one of the greatest works in the medium's history is little more than fan-fiction. And we know how they feel about that, don't we?

UPDATED: Richard seems to think I'm on shaky ground here. Read the comments to find out why.