Because that is exactly what Song of Ice and Fire is: A rather complex, nuanced political story along with of course the war aspect and the fantasy one but it's core is political.
The fact he brought Tallisa showcases just how much a suplified pile of dung the show is.
Robb made large mistakes in the past. It's not entirely out of character for him to bring Jeyene to the wedding. As for a "suplified pile of dung," that's subjective, and I'd rather not argue about the difficulties of translating literature to film across mediums.
But I will argue that so far, GOT and ASOIAF have acted a lot like alternate realities. Think BioShock infinite. Characters that did not die when they were supposed to, died later on, and characters that were added, also died later on. Her death in the RW might be foreshadowing another potential Stark being stabbed in his sleep.
As I've said, there are just way to many characters for the space of ten hours, especially in regards to Feast for Crows, where it's pretty much an 1000 page 'Foundation' novel. Perhaps that's why subtlety is severely lacking, some characters do not get the time they need, hopefully, in LF's case, they leave his moustache twirling self for private encounters with Varys, but in public, is the same old "you can trust me without a doubt, but keep one eye open" Littlefinger.
I was NOT impressed at all by the show's version of the Red Wedding. Where was all the chaotic fighting present in the book? Where was the tension as things went to hell and that? Where was the tightly packed hall?
Tension? I felt it the moment they brought Edmure out for the bedding. The fighting? They were all pretty much targeted, but yes, fighting should have been present, but it's hardly enough to make me disappointed. However, I do feel that the entire scene was focused on the utter abruptness of it all. And finally... that Hall was pretty small, unless your personal image of the setting was different.
It really was a very dull version of the Red Wedding which did not do it justice at all. Whereas in the book I could feel Catelyn's fear, anexiety, horror, terror and desperation and that meant I cared a bit for what happened despite my dislike of her and Robb as characters in the show I didn't give a damn at all.
That's a certain advantage, isn't it? We didn't get to hear Tyrion's personal "**** gold" joke and we don't get the magnitude in which he hates his father, nor do we get his symbolic dreams. Cat's feelings are impossible to convey unless you want some out-of-place Shakespearean soliloquy.