Some time ago, I started a thread about how Michael's greed for Roth's Havana empire clouded his judgment and blinded him to Roth's treachery. A big part of it had to do with your questions above.

Start with an even more basic one: Why didn't Michael question why Roth, who lived in Miami and had business interests in Nevada and Cuba, would interest himself in an obscure beef between the Rosatos and Pentangeli over three territories in the Bronx? Roth's interest was obvious: He was planning all along to have Michael killed--first at the Tahoe party and then, when it failed, in Havana. Allying himself with the Rosatos in NYC, and getting Michael to take their side against his own man Pentangeli, would pave the way for the Rosatos to take over NYC completely. Then the Corleone successors would no longer have the muscle-in-reserve in NYC that they could have relied on in case of a war or an attempt by others to take over their Nevada interests. Why didn't Michael see that?

My guess is that he might have questioned it at one point. And Roth, knowing how hot Michael was for the Havana deal, would have fixed him with his steely eye and said, "Michael, just go along with me--it'll be good for business."