Someone like Frank Trzaska could actually answer the question about why the sheaths are left-handed, but I'll venture a guess based on the time I spent in the military: sidearms were worn on the right hip. A pistol would almost always be in a holster on the right hip, so anything else (canteens, for example) would be worn on the left to balance things out. May have been the same idea for having the knife sheath made that way. I have owned a couple of the Catts, but never had the sheath.
"There are very few problems in the world that cannot be fixed by a suitable application of high explosives."