What did he say was the basis for such a move?
He didn't say. Maybe googling will turn up something on the powers of the Presidency wrt federal marshalls.
Aha. Have been googling:
"Today, the Marshals Service is responsible for providing protection for the federal judiciary, transporting federal prisoners, **protecting endangered federal witnesses** and managing assets seized from criminal enterprises."
http://www.absoluteastronomy.com/encyclopedia/U/Un/United_States_Marshals_Service.htm