Robb and Catelyn are supposed to suck, they were the two most retarded characters in the books. Pretty sure the point of Robb as a plot device is to show that he inherited all of the worst parts of Ned- idealism and ignorance, and none of the good qualities- honor, loyalty, and generally being a war hardened badass. It was all a game to Robb in the books, they dramatized it a bit out of context in the show so shrug.
You don't know Ned won't be avenged. If you're implying that you read the books you should remember that karma sort of punches the bad guys in the face throughout Storm of Swords, Robb and Cat not being the only bad guys.