I don't think the primary issue is what the OP had said about it being distracting. I think it's a copyright issue, as you say here. The copyright holders don't want their material to be distributed in images indiscriminately, and they have a legal right to demand that not happen in a private space.
Until fairly recently, I would say the past 8-10 years, theaters generally seemed to be very strict about taking photos in the auditorium, regardless of whether it was pre-show, during the show, or post-show. IIRC, Jordan Roth was the first major theater owner to realize that this was a losing battle and was generally fine with photos being taken pre-show or post-show. The curtain call is part of the show, and some copyright holders will care more than others. I'm actually surprised the OP hasn't encountered more resistance. I've been at several shows where ushers are shutting down picture-taking during curtain call, particularly if there is a curtain call production number.