Plagiarism is theft because it does take something away from the original author (attribution). Plagiarism and piracy are different concepts. Making a copy and forking the code is not what they did wrong, that part was authorized. Deleting the author's name and pretending it was their original work is the issue.

The idea that piracy is theft was not invented by the MPAA. I arrived at that conclusion myself, and indeed most people I've interacted with find it to be pretty reasonable. It's only ever been a minority of giga-nerds who try to claim that "stealing" cannot cover situations involving a non-scarce resource.