First lets look at the definition of theft:
1. 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.
2. The thing stolen.

I think it is clear that definition 1 is the one we want so lets look closer at that. And luckily for us What theft means in a hot topic in the legal scene becouse of 'joyriding.' so it gives us a large poll of people neutral on the copyright issue that have defined theft. Lets take a exert from one of them:

"Common-law terms such as theft that appear in the aggravated felony definition should be interpreted according to their "ordinary, contemporary, and common meaning." The definition of theft universally requires as an element the intent to permanently deprive the owner of property, or to approximate a permanent deprivation, under common law, the Model Penal Code, and generally under state law. To support its conclusion that joyriding can amount to theft, the majority decision relied not upon common law sources, state laws, or treatises on the definition of theft, but on a particular federal statute that relates to taking stolen cars across state borders. This ruling therefore appears to be in error. If theft in fact must involve a permanent taking, a conviction under a statute that allows conviction for temporary or permanent taking would not qualify as an aggravated felony under a theft theory. Such statutes would therefore be divisible: the portion that forbids permanent taking (or taking with intent to permanently deprive the owner) would constitute "theft"; the portion that is violated even if the intent is only temporarily to deprive the owner, i.e., where there is a borrowing only with intent to return, would not constitute "theft." The record of conviction would be consulted to determine whether the conviction was under the permanent or temporary intent element. If the record did not specify which, the INS would not be able to sustain its burden of proving "theft" by clear and convincing evidence, and the defendant would not be deportable."-- Aggravated Felony Convictions by Katherine Brady and Norton Tooby

Some say thare needs to be a intent of permanent deprivation of the property others say a intent of temporary deprivation of the property will do. Tittle 17 doesn't mention proporty once but lets say copyright is in fact proporty for the sake of argument. Now in your aledged stealing thare was no intent to deprive the Owner of their copyright, the Right to Copy, ether permently or tempory, and the Owner was not deprived of the copyright for they may still make all the copys they want.

"If nature has made any one thing less susceptible than all others of exclusive property, it is the action of the thinking power called an idea, which an individual may exclusively possess as long as he keeps it to himself; but the moment it is divulged, it forces itself into the possession of every one, and the receiver cannot dispossess himself of it. Its peculiar character, too, is that no one possesses the less, because every other possesses the whole of it." - Thomas Jefferson , 13 Aug. 1813

It could be said that Right to Copy a work is the right of all of socioty once the work divulged by its Author and only leased back to the Author at the cost of divulging his work to all of socioty, for with out copyright everyone has the right_to_copy and even copyright laws gives it to the whole of socioty for a majority of the time. So it is only you(plural) that even have then posibility of being the copyright thieves. For it is you(plural again) that intent of permanently deprivation anyone of the right to make copys.