I guess they have the right to reply, if they dont hide behind 'legals'.
I guess people don't dig too deep when there's a cash cow paying out.
Way beyond my area of expertise but from a couple of pieces I have read (and its also popped up on a couple of podcasts) whilst the publisher etc will do some due diligence there is a massive legal responsibility in any contract that the author signs (with both publisher and I suspect film makers) that what they are writing is broadly true, I therefore suspect that if it starts looking like its going to cost people money these contract requirements will be activated sharpish.
FWIW I assume the author is pretty lawyered up now anyone considering the working of their 'rebuttal' statements etc, which have been prime examples of saying something but actually not really anything!