[raincom]

Collusion among judges, lawyers, and prosecutors helps explain this phenomenon. This collusion occurs due to "revolving door". None of these parties (judges, lawyers and prosecutors) receive any bribes--so it is legal. However, the system is set up in a collusive way: lawyers for defendants become judges; prosecutors become a big law partner, representing defendants; etc.

[AcerbicZero]

This is likely a big part of it; take a trip to any traffic court in a medium sized town early in the day and you're almost certain to see some exceptionally chummy lawyers and judges hanging out.