The definition of plagiarism isn't connected to laws, so yes, it is. Intentionally taking someone else's work and passing it off as your own is bad juju through and through.
And even if I kept my code under copyright with an attribution license, not all forms of plagiarism would necessarily violate the copyright. For example, ideas and facts generally can't be copyrighted, but one could imagine cases where they can be plagiarized.
So even if I did use a license that requires attribution, that only works insomuch as plagiarism is a violation of copyright. And as we all know, there is a ton of grey area there. But regardless of the grey area, the plagiarism itself is still unethical.
And even if I kept my code under copyright with an attribution license, not all forms of plagiarism would necessarily violate the copyright. For example, ideas and facts generally can't be copyrighted, but one could imagine cases where they can be plagiarized.
So even if I did use a license that requires attribution, that only works insomuch as plagiarism is a violation of copyright. And as we all know, there is a ton of grey area there. But regardless of the grey area, the plagiarism itself is still unethical.
Some interesting bits are here: https://www.plagiarism.org/blog/2017/10/27/is-plagiarism-ill...