He is supposed to defend constitutional principles and nothing else.
It is hard to believe, but apparently he does not understand that. He sees himself as playing a role: the successful Chief Justice. He has read his history, and he knows that successful Chief Justices are those who get along with the other justices. Protecting American citizens from thuggish government overreach will not enhance the court's collegiality, so it is not a priority.
Roberts is like a savant. He has a superb knowledge of complex legal issues, but he does not know why he was hired.