The level of proof required for a conviction on criminal proceedings must be 'beyond all reasonable doubt'. Somebody has to test that evidence to make sure a degree of doubt cant be introduced. Thats the defence lawyers job. If the prosecutions case is sound, then a conviction should follow. That's the theory atleast. Reality can be a little different. But procedure is one thing. Folks moral values and their conscience is a whole different ball game.
We think of defence lawyers with disgust getting guilty folk off. But when we all burst the legal speed limits, we feel we should get away with it, because, well just because we think we should. Human natures a funny old thing.