The obvious answer to my question is that it's another of the many director/script "liberties" (or "irrationalities" if you prefer) that FFC and Puzo took to move the film along--in this case to set up Pentangeli's betrayal of Michael, the Senate hearing and all the great drama that followed.

But (and I admit, this is a stretch): Michael, the master manipulator, may have been testing Frankie's loyalty. Sure, he knew Frankie wasn't responsible for the Tahoe shooting. But Michael wanted more: he wanted Pentangeli to put his head into the lion's mouth by meeting with his arch-enemies, the Rosatos. If the mission succeeded, it might have made Roth relax and not get suspicious of why Michael didn't immediately have Pentangeli whacked. Then, as he told Frankie, he'd be able to string Roth along and find out who was the traitor in his family. And then he'd have Roth killed. A very convenient scenario for Michael, but it made Michael relax, and ignore the possibility--perhaps the probability--that the Rosatos would try to kill Pentangeli, leaving his NYC muscle operation in his enemies' hands.