[quote=Protagonist]
Sorry. To answer your question, it's because at this point in time, the world hasn't rid itself of due process. Maybe if we make some changes and try to justify the Judge system in a 'modern day' world. Maybe they're renegade SHIELD agents or something turned vigilante?
[/quote]

They are least likely thing to be renegades, they live and breathe the law. The simple option is to just have them side by side as a law enforcement agency.