Does anyone know why Honda chose to leave about bottom 6 inches of the negative cable bolted to transmission bare, no insulation, with just the stranded copper wires exposed? They seem to do this for all of their cars.

 

It's the ground wire, so no need to insulate it- at least from an electrical perspective. Anything it could touch should also be grounded. It's also a lot more flexible without insulation.
From a corrosion standpoint (esp for road salt), I do wish it had been coated!