Keep in mind the plugin has no knowledge of where the player spawns, only that they spawned somewhere. The 'on kill' option is there for jets because if some random player goes about their business on the ground we don't want the plugin caring about their jet loadout. However, if a player spawns then kills with a jet, it will scrutinize their jet loadout until they change it. It classifies them as a jet player whenever they kill with the jet.
There isn't really a way around this either because if you just admin kill a person when they kill with an invalid loadout in a jet, they just spawn in the jet again and kill with the invalid loadout again. Admin kills don't give deaths on player stats so there isn't a punishment there.