I recently bought a copy of Pride and Prejudice - Penguin Annotated Edition, and the author of the forward/introduction had the decency to indicate a few paragraphs in that subsequent discussion would include plot spoilers and recommended first-time readers return to read the rest of the essay after finishing the book so as to not be spoiled. I thought that was a great move.

I have the copy of F451 that you mentioned. I think there's a supposition (not a good one!) that the reader has already read the book, so the introductions are written more as an analysis/response to the story as opposed to a "let's get you oriented with the place/time/era in which this text resides."

I'm sorry you were spoiled, I hope you read the book anyway and enjoy it. It's a good one!