Turnbull beat me to it...I was writing the same thing about why Michael would stick around.

After the Superman club, Michael had to stick around for an alibi, and who better than hanging out with the true power behind the Batista government -- the Senate contingent and the big money interests. If Roth turns up dead, Michael's got those guys to vouch for his whereabouts, plus Geary was in his pocket. And I doubt Batista would remain loyal to a dead guy over the monied interests; it's not like Roth will be able to line that Swiss bank account.

But Batista resigned, and Michael figured it was every man for himself (he's out the door before the speech is over). Batista's out of power, and he presumes (incorrectly) that Roth is dead, so why not use the vacuum to make it safely off the island.

Bussetta was probably on his own after Michael gave the nod to hit Roth. It wasn't until Michael made it back to Las Vegas did he learn that Bussetta was dead.