This sounds to me like a case of fundamental attribution bias. We have no idea what was going through his head at the time. We're all human, and we all make mistakes sometimes. It's easy to just assume that the pilot is an incompetent decision maker, but it's way more likely that he or she just overreacted (or maybe wasn't given all the data needed to make a good decision)
While I agree, and I think the treatment of the passengers in question was idiotic, keep in mind that he did not talk to the passenger in question directly.
He was forwarded the request by a flight attendant or possibly the purser. The purser might have been forwarded part of what he said indirectly as well, from the flight attendant, and both might very well have exaggerated the story in part to explain why they haven't just dealt with it, but are coming to the captain with it.
It's perfectly possible that what the pilot hear from the purser was: "Some passengers are getting irate [he got others to speak up in support] because of the movie choice and has been pestering the flight attendants about it and won't give up. He's getting angry, and wanted to talk to you."
Might still be an overreaction, but a very different scenario than what the passenger claims (whether or not the passengers description is correct).
I'm sure the airline have "had a chat", but I also think they'd rather take the occasional cost of
a diversion like this than risk having a captain be too cautious in a situation where there's a genuine risk.
