Since they are bonded, why is it the equipment ground bus says 'equipment ground only'?
It does not always say that.
Years ago someone told me an inspector told them they could do this. I have never done it , by the way. But if ground and nuetral are bonded/married in the panel, I am not sure why it is wrong. Something to do with path of least resistance in case of a short?
The purpose of bonding the grounding electrode system to the neutral at the main panel is to provde a degree of protection against objectional currents. (say from electrical storms) And to insure a low resistance path for current to flow back to the transformer in the event of a line to ground fault.
Current does not take the path of least resistance. It takes all available paths, and shares them based on resistance. Electrical current is deadly because, even though the path through the human may have a very high resistance, the ammount of current it takes to kill someone is minimal.
If we look at a simple electrical circuit we can understand why we do not want to bond the ground to neutral at any other place than where it is required by code for safety.
Lets examine a circuit with one hot, one neutral, and one ground. Under normal conditions the hot is carrying current to a device and the neutral is allowing a return path back to the transformer. The ground is connected to the device box, ground prong, outer metal casing of equipment and such, just sitting there waiting in case it is needed.
If the neutral were to become open between the device and the panel, the device would quit working. The hot side of the device would still be live, but the user would examine the problem or call somone to come look at the probem.
If the ground and neutral were bonded at the device and the neutral were to come open, then the ground wire (which is bonded to the metal case of the appliance, the device box and etc) would now be a current carrying conductor. It would be carrying current back to the panel but the user would have no idea that there was a fault. Touching any part of this now live grounding system would allow the current to share you as part of the return path.
Again it takes very little current to kill a person. So, while the resistance of the path between you and the transformer would be very high, under the wrong set of conditions this situation could be deadly.
The same deadly situation could happen if the neutral were to open between the main panel and the transformer, but the added safety of having the bond at the main panel outweighs the risk of it being there. In addition, when the main neutral fails, there are usually several indications in the system that will cause a user to examine the problem, or call someone. (lights get dimmer and brighter, things only work when the oven is on etc.)