They weren't pardoned, just commuted to life in prison. https://apnews.com/article/biden-death-row-commutations-trum... "These commutations are consistent with the moratorium my administration has imposed on federal executions, in cases other than terrorism and hate-motivated mass murder."

He campaigned on ending the Federal death penalty, because it was cruel and impractical. In particular, toward the end of the previous administration, a rush to execute seemed to lack civil honor or religious circumspection.