According to webster, theft is:

a : the act of stealing; specifically : the felonious taking and removing of personal property with intent to deprive the rightful owner of it
b : an unlawful taking (as by embezzlement or burglary) of property

Abstrackt was referring to the definition of (a), which specifically mentions 'depriving' the rightful owner of their property. In the case of making an 'unauthorized' copy of digital data, no-one is being deprived of anything, unless you consider the opportunity the owner had of selling that item to the copier as 'property'. In many cases, the copier/pirate had no intention of buying the product in the first place, so there's no deprivation going on anywhere. ...but maybe you already knew that and were just being facetious.