Welcome, UJ! Hope to see many more thoughtful posts from you.

I believe this was a result of timing and quid pro quo. Vito learns of Sonny's assassination and immediately orders Tom to set up a meeting of the heads of the families. Michael learns about Sonny at about the same time, and Don Tomassino advises him to leave his villa subito. I'm guessing it took Tom weeks to set up the meeting, during which time Michael was still fair game for Barzini. In effect, Michael's life was held hostage by Barzini until he got what he wanted from Vito: agreement to protect the drugs traffic. Only then did he agree to call off his dogs.