The location of the injector nozzle is more significant in a two-stroke because of the length and complexity of the intake tract. From an emission control standpoint, one of the problems all engines have is in fuel separating from the fuel/air mix and running along the surfaces of the intake in liquid form, resulting in some fuel left unburned in the exhaust. In a four-stroke motorcycle, there is very little difference in distance between nozzle and combustion chamber whether the nozzle is at the throttle body, port, or in the chamber, and very little opportunity for fuel separation. In a two-stroke, where the fuel must pass through the crankcase, relocating the nozzle to the combustion chamber or transfer ports would shorten the distance considerably. Then, you would have only the matter of lube oil in the combustion chamber to deal with.