Hadn't Roth already made an attempt on Michael's life (Tahoe)? That would have certainly required the pre-approved go-ahead from the other bosses, and since that plan failed, I don't think Roth would have had to ask the other bosses a second time for approval to have Michael killed.

IMO, Roth was attempting to execute the perfect end-run by having Michael come to Cuba and have him assassinated as a result of "being in the wrong place at the wrong time," i.e. being in the limo when the government and the rebels had a clash. If that plan had been executed as planned, the other mob bosses likely would not have questioned it, even if they hadn't given approval a second time. After all, it appears they all wanted him gone, as it would mean more of a slice of the pie for them.

Just my two cents.