The trouble with using the word "theft" while making an argument about unauthorized copying is that it never holds water - does the popular singer lose their ability to sing? If not then the closest you can really get and still come across as genuine is to say they "stole" the uniqueness of their talent, rather than their unique talent itself
The above comment wasn't a passage in a legal document, it's a comment on a discussion site. "IP theft" is a colloquial, non-legal term generally referring to violations to copyright law. Neither the RIAA's complaint nor the DMCA contain the word 'theft'.
Well sometimes though. In the case of the Computer Fraud and Abuse Act the word “unauthorized” can mean basically anything the DOJ wants to pull out of their asses and prosecute. Fortunately we have courts to remove ambiguity.
The idea of copying as theft isn’t new but I agree it’s not really the same.