The Russia collision and DOJ/FBI abuses are two separate matters. One deals with investigating outside foreign collusion while the other deals with potential DOJ/FBI violations of Federal law and abuse of power.
Sessions is obligated to investigate abuse/violations within DOJ/FBI regardless of Rosenstein or his recusal of the Russia matter. What Sessions is doing in an internal AG matter.
Mueller is free to investigate but the DOJ/FBI can’t violate the law in the pursuit of their duties.
Sessions is recused on all Clinton matters and any mater connected to them. Emails, Uranium One, FISA abuse based on the Clinton funded dossier...etc.