It's suicidal for Robb to cavalierly break oaths like that, and contrary to the Stark character. It's the reason nations don't discard treaties on a whim; it means your word has no value. In-show, he'll get pretty much what he deserves.
Breaking a betrothal isn't quite as severe as breaking a treaty, and isn't that big a deal. No one complained when Joffrey broke his betrothal to Sansa, after all.
And a couple thousand soldiers isn't wroth a queenship. Other bannermen were contributing similar number. If it were tens of thousands of soldiers, maybe.
Furthermore, Robb could have taken the Twins with relative ease, he just didn't have time for a siege because he wanted to save his father. Walder didn't give Robb anything that Robb wouldn't have gotten after a couple months of sieging.
The Freys aren't royalty, and any political marriage is better suited to a royal or a paramount lord. That means that from a purely political perspective Robb was better off going for Shireen Baratheon, Myrcella Baratheon, or the daughter of one of the other Paramount Lords, perhaps Margaery Tyrell (Cat should have pounced on her as soon as Renley died, but really wasn't in a state of mind for political intrigue).
Nah, no one expected Robb to actually go through with it. The surprise was that he married someone of even lower station than his betrothed, instead of going higher and using the marriage to cement a claim on the Iron Throne. Now that was a slap in the face to Walder.
And if I remember correctly, Robb didn't make an agreement to marry the Frey girl. Cat made it for him. So technically he isn't breaking his word.