[simple man]

Originally Posted by JohnnieB

Blindness, explosions and acid baths come to mind, but there may be other reasons also.

The battery pack contains several Megajoules of energy, more than enough to create an arc many times brighter than the Sun.
Hydrogen gas is produced during the charge/discharge cycle and all that is needed is a spark to go BOOM!
The electrolyte is 35% H2SO4 and there are gallons of it waiting to be launched.

If B- is attached to the frame, any metal to metal contact between any of the high current carrying cable connectors and the frame could:
A. Weld the short circuit in place
B. Explode one or more batteries
C. Spray gallons of Sulphuric Acid all over everything in the vicinity.

While isolating the frame doesn't eliminate all the safety hazards lurking under the seat, it does mitigate some of the avoidable ones.

When working under the seat, always wear safety goggles, full face shield is preferred.
Avoid using wrenches long enough to short between battery posts if dropped. (Torque spec for battery terminals is only 100 In/Lb, so you don't need much leverage)
EZGO and other cart Manufactures advocate using insulated wrenches. (Personally, I believe insulated tools tend to create a false sense of security)

Thank you so much! I found another reason last night! It may prevent the nasty shock from having your sweaty arm touch a live terminal while laying on the frame!