He resigned for being wrong about that. And then Dubya surrounded himself with Nixon's unindicted co-conspirators and resurrected the notion, which cons continued to defend until Obama got elected. And re-elected.
Apparently if the president executes American citizens without a trial it is legal now. according to Obama's co-conspirator AG Holder, due process is now the president ordering a firing squad. But ah heck, Bush forced him to do it.