They are officers of the court and can not present testimony or theory that they know to be untrue so they are very keen to extract the plausible ideas from their clients why they might not be guilty and start from there.

As for OJ...I have to believe that he has never confessed to any one save for maybe Al Cowlings or Roosevelt Grier and for obvious reasons, they aren't going to say anything. I think in his situation, it was extremely important for his lawyers to believe that he was innocent and could not have put for the defense they did if he had admitted guilt to any of them.

People can (and do) say many things about lawyers, much of which may very well be true but a lawyer defending a client that he KNOWS to be guilty can only be effective at minimizing the conviction, negotiating a plea arrangement or be an effective agent in sentencing hearings since the guilt is by then, assumed.