Why are the ground and neutral bonded in my panel?
According to the North American electrical code (US and Canada), Neutral and Safety Ground must be bonded at the service entrance (or main breaker panel).  The green wilre will also be connected to Earth via a ground rod driven into the earth near the service entrance. Neutral will also be grounded at the power pole and distribution transformer.

All this grounding ensures that the AC wiring is kept near earth potential, and won't drift up to 12 KV due to a fault in the distribution transformer.