A good question. But it's my understanding that Michael never actually took sides against Pentangeli. It was all part of an elaborate ruse to con Roth into believing that Michael was on his side, so that he could flush out the identity of the traitor in the Corleone family. Am I missing something?

As to the question of why Michael never smelled a rat, is it possible that he was simply going along with Roth on the Cuban deal, at least partially to get a better view of what Roth's real intentions were in the Bronx?

~ Q