ok. since no one yet seems to understand why we only fuse power wires, let me explain.
first, lets all recognize that the convention for DC current flow is backwards. DC current "flows" from negative to positive. ok, let's move on.
Regardless of current (charge flow) direction, we still recognize that power wire is fused to provide two things:
1. over current protection
2. short circuit protection
The DC electrical system in a car uses a common ground - i.e. the metal in the car. This provides a convenient ground for all of the various electrical components in the car, saving money and weight.
In order to protect the battery, only one of the wires needs to be grounded. We fuse each power wire because those are the wires that are dedicated to each load. If you fuse the battery ground lead, it would be a ~300A fuse so it could handle the starting current as well as every single piece of electronics in the car. let's think about what that would do - it certainly wouldn't provide over current protection for anything, and various wires in the car would melt before the fuse blew due to a short (assuming that was the only fuse in a circuit). If you fuse your ground in addition to fusing each power wire - the only thing you will accomplish is increased voltage drop. the positive wire fuse will certainly blow first, unless of course that wire is the same ampacity as the ground fuse (i.e. for 1/0 wire or something).
We size the fuse for each power wire based on the wire size, and we size the wire based on the ampacity (load) of each device that wire serves. positive wire fuses can be selectively sized. if you were to fuse each ground wire - it would be between the electrical device and the metal on the car. that would be effective but annoying.
you want wires fused as close to the source as possible - in this case the battery positive terminal or fuse box. by fusing the power wire you are protected from any short circuit that happens on the load side of the fuse. if power wire comes in contact with the car metal, the circuit is completed, current leaves the negative terminal - flows through the ground wire to the chassis - contacts the short location and travels down the power wire trying to return to the battery - but it first passes through the fuse which opens the circuit - and the battery is protected.
fusing the ground is pointless due to the size of fuse required... and it's redundant since the power wire fuse already protects the battery. you only need one fuse in the DC circuit.
lastly, the only fuses that are directly connected to a battery terminal are inherent to the battery terminal. Streetwires has some that use AFS fuses. otherwise, you still need a protective fuse holder and a short lead of wire to the battery terminal.
you only need one fuse in the circuit and fusing the positive wire is sufficient. fusing a ground wire won't hurt anything unless it blows while starting the car and you're left stranded.