As AG, it’s his responsibility to ensure laws are enforced. Rosenstein has an obvious conflict of interest in any potential re-investigation, and therefore Sessions willingly passing the responsibility back to him is a dereliction of duty.
He should have appointed a separate special prosecutor, which would still allow him to satisfy his want/need of personal recusal. The fact that he didn’t smells like a oontinued coverup of the issue.
I noticed that you correctly used the word "re-investigation".
Mueller's FBI investigated the deal for four years. And then the FBI investigated after that to, up to the point where Rosenstein indicted and gave easy plea deals to a couple of low level operators.