Many guns don't have a safety because the manufacturer never bothered to add one. Reverse triggers don't have a lot of space to incorporate a safety in any case as the sear lever sweeps through the location immediately behind the trigger where a safety would usually be applied. It is possible to mount a safety in the trigger finger guard frame such as is used in the MVD guns. If you dive from boats and are keeping away from fellow divers then a safety may not be necessary, but diving around reefs both near and slightly offshore it is possible to encounter other divers who you do not know. If those other divers start to follow you then you would hope they have safeties on their guns and use them. A following diver has the potential to shoot divers in front through being careless and not concentrating on what he is doing.