The Rosato Brothers represent just how thoroughly Michael's greed for Roth's Cuban gaming empire blinded him to Roth's treachery:

In a nutshell: Logically, why would Roth, who had no interests in New York, involve himself in a dispute between Frank Pentangeli and the Rosatos? And why would he take the Rosatos' side? Michael should have smelled a rat. I'd guess that he might have probed Roth about his interest in them, maybe not have been convinced. But then Roth, playing on Michael's greed, said, "Look, Michael, just do me this favor. It'll be good for our deal." That was all Michael needed to hear.