The Bush administration tortured.
Torture is a crime.
President Obama took an oath to "faithfully execute the office of President of the United States," and to "preserve, protect and defend the Constitution of the United States."
Criminals must be prosecuted or pardoned in order to uphold the rule of law.
President Obama must prosecute the Bush administration or pardon them. Failure to address the violations of U.S. law would be a violation of his Presidential oath of office.