I agree with the thrust of this article, that norms and what we perceive as good or desirable extend considerably beyond the minimum established by law.

But a point that was not made strongly, which highlights this even more, is that this goes in every direction.

If this kind of reimplementation is legal, then I can take any permissive OSS and rebuild it as proprietary. I can take any proprietary software and rebuild it as permissive. I can take any proprietary software and rebuild it as my own proprietary software.

Either the law needs to catch up and prevent this kind of behavior, or we're going to enter an effectively post-copyright world with respect to software. Which ISN'T GOOD, because that will disincentivize any sort of open license at all, and companies will start protecting/obfuscating their APIs like trade secrets.