Interesting question, TB.

I think that Michael, once he calms down, does not view a terrified Fredo as a continuing threat. He also does not believe that Roth/Ola would have told Fredo anything important that Michael does not already know. After all, Michael has learned for sure who his enemy is, and who they had on the inside.

Michael didn't realize that Roth had a Plan B (the perjury trap) ready to go, so he thought he had the upper hand, and that any future plans by Roth wouldn't involve Fredo. And if Roth did use Fredo again, it would be to Michael's advantage to lure Fredo into a false sense of security. You know, keep you enemies closer.