No requirement doesn't mean "don't be a mensch"
I'm not, and don't have any intentions to monetize it.
They chose -- explicitly, voluntarily, knowingly -- to give the thing away for free. That's their right for their own creation. Your preferences don't supersede theirs.
By profiting off their work and not supporting them you are making your own choice. It says nothing about the authors' preference.
There are many other licensing terms in common use. They chose that one.
That's fine, they can do what they like with their work. There is no moral obligation to reinterpret their motives in a fashion that differs from their formally expressed preferences.
They chose to accept donations -- also fine. If you want to donate, go ahead. If you don't -- that's fine too, by definition.