It could be a cultural thing. On red sea trips I have always paid 0% for it as it been free. It makes the live of the guides easier since everyone will be basic

It could be a cultural thing. On red sea trips I have always paid 0% for it as it been free. It makes the live of the guides easier since everyone will be basically on the same mix and having an accident while out on the sea is bad publicity, bad economically, and bad for the tourist. Dive centers on land tend to ask money for nitrox, but I don't even remember when I last saw someone choosing air unless we knew that the maximum depth will be around 5-15 meters where it becomes almost impossible to reach ndl on a single tank.