^^
As Holden said, they are deeply involved in the Canterbury and Donnager incidents, Holden's face is everywhere, their only option at the moment was Johnson, Holden had to cut a deal, plus he feels responsible.
None of the crew comes from Earth except Holden who left for its own reasons, they can't go back on Ceres, too hot of a place now, Mars is out of question and Earth not even an option.
Johnson isn't a "terrorist", he was labeled as The Butcher when he served in the U.N military after the Anderson station destruction. Now he is OPA, people that want their independence ( just like Mars is a Republic in this present ).
Edit : everything is linked to what happened on Phoebe, references in ep04, the exchange between Holden and the Martian Lopez, ep06 when Miller recovers the file from the data broker and shows it to his boss.