Fair-ish point. I was considering plagiarism in the context of computer programs, which is -at best- dreadfully difficult to do without also violating copyright

Fair-ish point. I was considering plagiarism in the context of computer programs, which is -at best- dreadfully difficult to do without also violating copyright and/or software licenses. I'd also rephrase your first sentence as "Plagiarism isn't illegal, actually.". Unless you're rich and/or very influential, plagiarism is a seriously bad thing to do.