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The Boltons and Freys are doing this under the Lannisters’ implicit protection. Roose isn’t an idiot and knows that the Red Wedding would be condemned and seen as an atrocity. He also probably suspects that Tywin would throw them under the bus if it came down to it in order to distance himself from what happened, if he’s cornered on it. Implicating the Lannisters openly at the  The Mask of the Red Death 3D face mask time of the Red Wedding means Roose is getting out ahead of a potential backtrack from Tywin. If Tywin claims to have had nothing to do with it, pretty much everyone in the vicinity of the Twins, on both sides, whether they witnessed it directly or not, would know Tywin was full of shit. It’s Roose’s insurance policy: If anyone eventually tries to gun for him for this, they’ll have to go after the Lannisters, too. He’s not letting the Lannisters get away clean after the Freys and Boltons do their dirty work.