Also IIRC stripping a copyright notice from plagiarised work is a more severe offense than just the plagiarisation alone.
Plagiarism is an ethical offense but not a legal one and isn’t limited to direct reuse. It includes using ideas (even in your own words or representation) without attributing them.
Copyright violation is a legal offense and applies to direct reuse (including various forms of “remixing”).
(IANAL, this is not legal advice.)