I was more surprised by the casual brutality of Bolton. He's been presented, thus far, as a mostly loyal, ho-hum character. I mean, he let Jaime go secretly and that was a bit alarming, but he's hardly been portrayed as a ruthless turncoat waiting to happen. Frey has always been shown, at least to some degree, as a tentative/undesirable alliance. I read in a review that the salt and bread thing was highlighted early in the episode because that was an ancient Westeros tradition meant to ensure safety against betrayl. So not only did Bolton and Frey betray them, they basically took a big steaming dump all over some very engrained traditions in doing so. I love the nuance and the brutality of it even more on the second viewing.