To quote myself:

There are times I have no urge to spoil books or movies. They are not frequent but they happen; usually it is a sign that I am enjoying the media, or that it simply doesn't matter to me much what the plot actually does.

For what it's worth, I frequently watch horror movies (if well done, capable of building great tension) and almost never spoil them. This is honestly usually because I don't preplan what I'm going to watch, I just throw it on, and then I sit back and enjoy the movie until it's done. I don't spoil things all the time by any means. I tend to start gently batting at my movie companions as the tension level rises. They usually find it amusing. I get that thrill and understand it. I think generally that horror seems to fulfill this urge in me and outside of horror, I don't want it. Just surmising.

I would also say that the interpretation that I spoil things and then, instead of focusing on plot, focus on storytelling and technique, is not really accurate. I still focus on the plot and what's happening. I may know the broad strokes when I have spoiled something but I don't know the details and I care very much about details - I am one of those people that watches everything with subtitles on because I hate to miss even a single word, even though single lines of dialogue really are not that important to one's overall understanding of a movie. But I think knowing the broad strokes enables me to pay more attention to the fine details of the plot.

To show myself:

I am what I am, and I know what I am. I embrace my nature.

P.S.: I have some favorite bad movies. And I find that discussions about media that you dislike - be it a book, a movie, or whatever - can actually help you realize more quickly and with more understanding both what you like, and what in general makes examples of that media strong, or weak. Book club discussions are awful when everyone likes the book. There is nothing to say. It is when you have differing opinions and complaints that you really get into something.