By American on average want TV shows not to demand too much of its audience, but it makes for fast food TV at times too.

I think they put enough emphasis on a detail like this. They devoted a whole scene to provide to setting it up by having Cirsei explain the whole story and mention the song as she was issuing a memorable warning and a death threat to Joffrey's bride. The episode is named after the song to remind people of last week's scene. In editing they put a lot of emphasis on Cat realizing something is wrong with the music being played. To dispel any doubt/confusion about what attracted her attention and worried her, they cut to the musicians (who played badly... They were not pros), then there's the body language of Cat and Roose. The audience isn't supposed to recognize the melody, it's supposed to puzzle out what alarmed Cat, and at the latest when Roose sends the regards of the Lannisters realize the music that Cat recognized was "Rains of Castamere". The great acting did the rest - everything was decipherable on Cat's face, and Roose's. This begged for a bit of subtelty, only Cat in the room figured out the implication of that song being played, it should dawn slowly on the audience too, and the slow pace of that scene reflected all that.

Martin had his reasons for having the whole thing happen offscreen (and far from characters who would have stopped Robb) and for having Robb risk repeating the sin of his father who supposedly sired a bastard. So Robb married Jeyne, whom he had defiled The whole thing was annoying and not very dynamic, as Martin wanted. He had no need for the relationship itself, unlike the TV series that needed material for Robb.

The TV show needed someone more like Talisa to provide an on-screen confidante to Robb that woul be neither Cat or an experienced bannerman and justify at the same time that Robb made such a stupid mistake. They weren't so succesful with the character (she stood out way too much as not a GRRM creation) but they needed someone who could follow the army and move around the camp without being sheltered in a tent with handmaidens and talking only to Robb and Cat. Thus the nurse... But they needed someone with more sophistication than a regular camp follower (usually whores, beside), so a foreign high born Robb could marry she was. I'm guessing the main reason for not making her a "more interesting" Jeyne was her background. They wanted her dead at the wedding, not to be stuck with someone the audience expect to keep following if she survived. Yet, they might want to include Westerlings later - they don't know this yet -, and the murder of Jeyne at the RW would have changed the picture. Changing her house would have been too obvious that she was destined to die at the RW (they wrote Talisa so we'd wonder what they were up to). Another reason is that Jeyne's background and story (eg: when, where how and why she ends up married to Robb) were too complex for the TV show (they didn't have time for this subplot that happened late and that would have been tedious and that happened off sçreen even in the book), with no real pay-off.

They also must have realized it would be much easier to get the viewers to sympathize with Robb falling in ve on screen (which was a reason for beginning early, in s2) than to have Robb pull a wife out of nowhere because in some offscreen episode he couldn't keep it in his pants and Stark honor demanded (and his dad supposed youthful mistake with lady added pressure) he married her.

I didn't think Talisa was much of a success, but i understand why they totally replaced Jeyne with someone who was more simple and practical for TV.