Well no not exactly what Maxine Carr did. What Maxine Carr did was cover up for Huntley then owned up and told the truth. She was gaoled for perverting the course of justice in giving Huntley an alibi.
They had a strong case against Huntley though, and didn't need to make a deal with Carr. With Jeremy, they needed JM's testiomony to add to the rest of the evidence.
It's not ideal that she got away with likely nkowing more but proving she knew mroe would be impossible without Bamber and he was never going to tell the truth was he. Bamber - who killed the family - was the main culprit and ensuring he was arrested and charged for his crimes was the main goal, they needed Julie's testimony hence a deal.