If you want to look up the end of a mystery that is your prerogative but that doesn't mean everyone else feels the same. It's not like people are forced to read the Wikipedia article before reading the book or seeing the play.
I've read a lot of Agatha Christie books and yes it would have ruined my experience if I was spoiled before hand. Because part of the fun of her books is trying to figure out who was the murderer.
And sure a reader shouldn't go somewhere where they know there is going to be spoilers but out of respect for someone who might not have read the book or watched the movie I don't go posting MAJOR spoilers (like the end of a mystery) without giving a warning. It's about having respect for others.