You believe you know better than the authors themselves about how they ought to be treated?

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.

You've emphasized this point more than once, but I don't think it's a good counterargument to the GP. It is equally valid to say that they have explicitly (voluntarily, knowlingly) chosen to accept donations for their work.

By profiting off their work and not supporting them you are making your own choice. It says nothing about the authors' preference.