In the first Godfather, Michael explained to Tom that he would be in on the legal side only; Tom was not a wartime consigliere and Michael knew that his agreements with Roth could turn into an all out war. But Michael knew that his meeting with Geary was very standard, nothing dangerous. You could believe Michael, that he was keeping Tom out of the heavy stuff because he wanted to keep him safe, but you can also come to the conclusion that Hagen was just never trusted fully by Michael. Maybe it was because he wasn't blood, maybe because he wasn't Sicilian, maybe it was because of his poor performance in the war with the five families while Sonny was acting Don. I guess we'll never know what Michal was really thinking.

However Hagen was made acting Don of the family while Michael was in Cuba, however, this seemed to be something just to keep Tom on his side (Michael was always a little paranoid that Tom would leave, even if he didn't trust Tom, Tom was a good lawyer and would prove very helpful to him). Besides Michael new that all business at that time had to deal with Roth and Ola and so Tom would not have to do much besides keeping Kay and the kids safe.