Don: Because it harps back to those days when conventional current was considered the done deal. The positive (Anode) is still considered the so-called "hot" connection (even more so in AC circuits), so fuses by convention are placed in series with the positive rail. Also, bear in mind that in most modern circuits the negative is mostly always tied to ground in one way or another. The negative connection of, say, your R/C reciever will also be tied to the negative, of, say, your servos... and your ESC and your LEDs, etc. So it makes sense to fuse the positive side only.
Incidentally, most folk, when measuring the current in their circuits, will mostly always connect their ammeter in series with the positive rail. Did you know that you will get a much truer reading of the total current draw if the ammeter is connected in series with the negative rail? A thousand Green Shield stamps to you if you can tell me the reason why.
Roy: Black wire syndrom is basically a chemical reaction of the negative copper wire coming into contact with air. It's even more noticable when the air itself is humid or damp - a form of electrolyosis, very similar to how tin plating works. Try placing a copper probe and a zinc or stainless steel probe in a solution of acidic/salt water, then connect both probes to a DC supply. After a while you will begin to see the aforementioned chemical reaction taking place in the form of bubbles collecting around the probe which is connected to the supply negative probe - not the positive probe.
The general advice for R/C modellers is to not store their models in a cold, damp shed. If you must store your models in a damp environment, then a simple prevention of black wire syndrome is to disconnect the battery/batteries altogether. Better still, it's recommended to remove the batteries from the model during the Winter months and keep them in a room at normal ambient temperature.