This is why you ground to the chassis and not the body.
All that noise, when the amps are grounded to the negative terminal of the battery, comes from the differential between the HU ground (body) and the amp ground (battery).
If you look at the amount of noise that can be seen at the battery itself from the car's normal charging and discharging you can find that it is anything but clean.
Another reason not to use the battery is safety. Now understand that this is something I've read and never seen, but hey.... If the strap between your car and the battery fails, as you've pointed out the flow will still find ground. What's in line with that? Your amps. Is this something that you'd ever see? Maybe not, but there is still no reason to risk it either.
At the end of the day you can ground to the battery, but in many circles it is not considered good practice.