I think it just depends on the context, and the type of class. Like I said, in my mind, it's just a courtesy, not something I would suggest mandating. Now that that professor has had the experience of a student fainting in class, he or she might consider mentioning it in the future before showing the video. I'm not suggesting that we all have to be mindreaders, or that everyone always has to announce to strangers and acquaintances all the things that might trigger them. I guess I'm just saying that there are better ways of addressing this particular type of experience outside of either "it's your problem, shut up and deal with it" or "everyone has to walk on eggshells and cater to every issue". I think those responses are simplistic and immature, and it's not absurd to look for understanding somewhere in the middle.