Lana, not everything in that scene was logical or even credible--FFC took directoral license to enhance the drama, as he did throughout the Trilogy. Keep in mind the broad themes:

1. Greed motivated both Michael and Roth: Michael, greedy for Roth's Cuban gaming empire; Roth for the $2 million and the opportunity to put Michael in the killing-bottle in Cuba. Greed makes people careless; or as Vito said, "Women and children can afford to be careless, but not men."
2. Corollary to the above: They underestimated each other: Michael, initially thinking Roth was a harmless old man; Roth, thinking Michael still didn't know he was behind the Tahoe shooting. Michael was the more clever because he knew, right after the shooting that Roth was behind it.

Those themes run throughout the Trilogy. Greed for money made Pauilie Gatto think he could get away with betraying Vito; greed for revenge blinded Carlo to the fact that his beating of Connie would identify him as Sonny's betrayer; greed for respect and status was Fredo's undoing. And, Vito underestimated Solozzo and Tattaglia; Sol and Mac underestimated Michael; Sonny and Tom underestimated Carlo; Carlo underestimated Michael; Michael underestimated Altobello; Vincent and Michael underestimated Mosca, etc., etc.

Keep in mind, too, that Michael brought the bodyguard, Buscetta, to Cuba in part to fool Roth into thinking that he still had no clue about who was behind the Tahoe shooting, hence he didn't bring Rocco and/or Neri. Roth and Ola, seemingly protected by Batista's forces, thought they had nothing to fear from Buscetta.