A good rule of thumb on those is 5-10 amps max, and that depends on the vehicle. I have taken them apart on a few vehicles to see what gauge of wire is in there, most of the time its not much heavier than 18 awg, if your lucky it might be 16 or 14 instead.
Why they fuse them at 20 amps? Well there is a few reasons i guess but i don't agree with any of them. A lot of the times its because the fuse is actually powering more than just that outlet, i don't know for sure if its this way on the KJ as i haven't looked at the service manual, but i know on our PT Cruiser there is a 20 amp fuse and that provides power to both the front and rear power outlet as when i pull that fuse they both go dead.
My usual habit is to pull the 20 amp fuse and put in a 10 amp fuse for greater safety, i want the fuse to go before wires start melting if there is an accident.
I was told once by a service manager that they put a 20 amp fuse in that circuit to handle large turn on current surges, for things like those coolers and warmers, etc. They have a large current draw for a few seconds that can be around 10-15 amps, but once they are running the only pull 5-10 amps continuous.