Nobody is there to protect the cop. I think the prosecutor should also do that. In the past, it was 'Wink, wink, nod, nod; here's what I want you to look at.' But that's why states have moved away from grand juries. You should also protect the people you're trying to indict.
A prosecutor usually wants someone charged when a grand jury is used. But it may be simply a matter of wanting public confidence that you're making the right decision.