The following tidbit from Obama Transition Team Member Explains Why Administration Chose Not to Prosecute Torture:
… if laws were enforced against Bush, Cheney, or their subordinates, the Republicans in Congress would retaliate by trying to block any useful piece of legislation.
provides an insight into Obama’s strategic thinking.
I would think that the threat of such prosecution could be used as a weapon to keep the Republicans in line. Instead, Obama thought that it could be used as a weapon against his initiatives by the potential targets of the prosecution. According to his biography, Obama decided to leave the field of community organizing to go to Harvard Law School so that he could learn where the real levers of power were. I think he wasted his money if Harvard taught him not to threaten to prosecute people because they might get angry at you.