Michael had said that Tom was the only one he could really trust and if we remember Sollozzo's line "...we know you're not in the muscle end of the Family...", so in the eyes of the authorities Tom was most probably fairly distant from the illegal goings on of the Family. Another point that occurred to me was that Michael was relaxed going into the Senate hearing as he didn't know about Pentangeli. His attitude may have been "They've got nothing on me, so no need for a big shot lawyer"