I think there might be cases where the ethical thing to do would be to respect an author's non-binding request. However the request in this case seems directly contradictory to the principles of open source software and thus I can't bring myself to see it as legitimate.
Edit to add, an example of a non-contradictory request might be to contribute monetarily in proportion to the financial benefit you derive. It's an additional non-binding request to help sustain the community which seems reasonably consistent with the ethos of opensource to me.
The issue is that opensource is a movement that comes with a set of values attached. The licenses aren't impersonal the way the copyright system at large is.
Edit to add, an example of a non-contradictory request might be to contribute monetarily in proportion to the financial benefit you derive. It's an additional non-binding request to help sustain the community which seems reasonably consistent with the ethos of opensource to me.
The issue is that opensource is a movement that comes with a set of values attached. The licenses aren't impersonal the way the copyright system at large is.