Firstly, isn't the point here that the PA was used at a captive audience, who have no choice but to listen. Should the same speech be made on a street corner, then we would all be free to walk away and ignore him, or stay and listen, as our choice dictates. That option was not available to those passengers.
Secondly, the subject matter is inappropriate for the situation. If the Captain has the right to concern himself publicly in these matters in this location, then I, as Cabin Crew, may choose to exercise the same right on the PA and seek to identify Democrats, Masochists and Anal sex practitioners, or to promote Satanism as a viable lifestyle alternative. I don't do this because (in western societies at least) this kind of zealotry is not part of mainstream life and is regarded by a substantial proportion of citizens as offensive or an infringment of their rights - also I'd get fired!.
Thirdly, the situation demonstrates a considerable lack of good judgement by the pilot involved. Nervous passengers might reasonably begin to question the quality of his judgement in other areas related to flying and become uncomfortable about putting their lives in his hands.
I'll be fascinated to see where this one goes. Initially, it will no doubt attract comment from all the various religous fanatics that the world has to suffer, and I'm sure someone somewhere will create a defense that waves the (soiled) flag of religous freedom and the right to free speech - where we go from there will be interesting!