My understanding is that kempo can be used in Japanese to refer to martial arts generally. But when people in the west talk about kempo it's usually a reference to the American martial arts schools that popped up in Hawaii and thereabouts in the 1940s/50s having been imported from Japan/Okinawa by people stationed there after the Pacific conflict. I think it's just one of those words that's taken on a life of its own in the modern West. A Japanese person saying it might be talking about kung fu, but a Western person saying it probably means American Kempo.
