"Justice" system?

I have a guilty pleasure: the tv show "Boston Legal", created by David Kelley, who is liberal in both the political and Catholic senses of the term. His politics often come out in the show, but generally they do not outweigh the other, more positive dimensions of the show. Simply put, the man knows his business, and he does it very, very well. He manages to create a program that is both very funny and simultaneously fairly substantial, whatever his own politics, apart from the more egregious instances of outright propaganda.

Anyway...

In tonight's episode, the law firm to which the characters belong represents a man who is by his own admission to them guilty of negligent homicide (he hit and killed a man with his car). Candace Bergen's character is the lead attorney on the case, and she succeeds in getting a not-guilty verdict for her client.

I just don't get that.

Isn't it called the "justice" system for a reason? How is it that defense attorneys can conscientiously defend clients whom they know to be guilty of the charges made against them? I've heard the argument made that this prompts the state to make its best case, but that's not what the justice system is about, is it? It's about the guilty being punished and the innocent not, right? So what's the justification for defending someone who is admittedly guilty of what they are charged with?

I'm hoping someone can explain this to me in a manner that doesn't make a travesty of the idea and virtue of justice.