Game of Thrones question (never seen or read it)

Even though I’ve never seen an episode of Game of Thrones (and don’t really have a desire to), there’s one question I have based on all the hoopla regarding the finale:

If it’s based on a book series, why are people so surprised (or upset) about the ending?

And if it’s so different from the books as to cause surprise - shouldn’t the difference be what people are talking about?

I clearly don’t get it - but I can’t think of another show (or movie) based on a book that would lead to this kind of thing.

Only the first five of the seven projected books in the Song of Ice and Fire series on which Game of Thrones (actually the name of the first volume) have been published, the most recent one in 2011, the same year the TV show debuted. I think when the show began it was assumed that the author, George R.R. Martin, would have finished the book series before the TV series ended, or at least be closer to the end.

Martin discussed an outline of how the story line progresses in the final two novels with the show writers, but they strayed from the plot of the earlier books so there is no way to know how close the ending they chose is to what Martin’s eventually will be.

The book series hasn’t been finished yet. So far only 5 of the 7 planned books have been published.

The show finished the published book material in season 5. Seasons 6, 7 and 8 went beyond what the books covered.

The TV show ran out of book material around season 6. The last 2 seasons have been completely new material not written by the author (he is a co-executive producer of the show however so I assume he gave some input).

The author has 2 books planned, but that probably will never happen.