That's their job.
* Legislators create the law. (legislative branch)
* Prosecutors sue for enforcement of the law. (executive branch)
* Judgement to use JUDGEment in applying the law. (judical branch)
If you have sucky laws or indescriminate judges, fix those. Don't cast dispersions on prosecutors for prosecuting laws they didn't write, for sentences they don't make.
Not maintaining the balance of powers is how you wind up with out-of-whack and mismanaged governments.
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EDIT: Prosecutors are the inverse of defense attornies. Why is the DA defending this scumbag of a human being?!?! Because that's their job: provide the best possible defense for accused criminals. Likewise, prosecutors provide the best possible prosecution. Denegrating either one for their role is misunderstanding their functions in the judical system.