Forking for whatever reason is in the spirit of open source. Asking if something is right is nonsense. From an ethical point of view, it follows the license of the project so there is no issue.
If the author of the original source had an issue with the above scenario, they would've chosen a more restrictive license that didn't make it possible.
> From an ethical point of view, it follows the license of the project so there is no issue.
This seems like the opposite of the usual ethical/legal distinction. It’s definitely legal to fork the project and the community, but not necessarily ethical.
I don’t think you can blame authors for not using a license that describes exactly what they are happy with. What constitutes an “ethical” fork is subjective, based on how necessary the fork is, and I don’t think a license could describe that.
Also, consider this analogy. The MIT license doesn’t contain a patent grant. So if some MIT-licensed software uses a patented technique, the software author and patent holder is within their legal rights to sue all forks of their for patent infringement. But this would be unethical, as the MIT license implies that the licensed software is given freely, not that you will sue all users of your software. Would you blame the sued parties for choosing to use MIT-licensed software, the way you are saying you would blame software authors for choosing a license that doesn’t forbid certain forks?
If the author of the original source had an issue with the above scenario, they would've chosen a more restrictive license that didn't make it possible.