I’ve always considered this a grey area and one I’ve always avoided. Forking a premium plugin seems unethical to me too. But as most are released under GPL, it’s tempting to think, “well, why shouldn’t I?”.
However, if I’d written a premium plugin which was my sole means of income, I wouldn’t be happy if someone forked it and started distributing it themselves. So, by the same token, it’s not something I would consider doing to other developers.
I think there are times when, even if something is legal, it may still be unethical and, in my view, is to be avoided, especially if it’s in any way associated with the CP community.