Japan was a naval power, being an island nation, so the fact it had a large naval air arm is not at all surprising. Japan, like the U.S., and Soviets, and others, had their air power controlled by their respective army and navies, with seperate air arms in both services. Most nations in WW2 did, nothing really unusual about it. The tradition of land based naval air forces goes back to WW1 where the German Naval Air Service and the Royal Naval Air Service, both operated land based fighter and bomber sqaudrons in support of the front. Naval air power was never limited to just ship borne aircraft, even after Carriers became numerous in fleets. Thats even true today.