For the simple reason that while you know your way around the .338 and would load suitable 300grain bullets, some numbnut would toddle off to Africa with 200 or 225 grain non-premium factory loads and then complain about getting stomped on... or a least leave some wounded buffalo running around the bush to stomp on natives. Don't forget that the rules were probably introduced with resident hunting in mind, much of which would be unguided. At least in the 9,3 the factory standard load is 286 grains which gives more margin for error even in non-premium bullets. I guess that's the theory anyway. Allowing the 9,3 was an exemption from the .375 only rules introduced - exempted only because of the number of guns in use and its proven reliability (hence the number of guns in use).