An irrational fear of suddenly using all of it up. Before they got their phones, we drilled it into them to be conservative in their data usage. It's not that they complain that they have too little data, or how annoying it is that they have to leave it switched off to conserve it, they somehow are convinced that it is pointless to leave it on. We have mentioned numerous times that we'd be fine with upgrading their data plan, but they don't want to. It's like us in the nineties dialing into our ISP to download e-mail. Weird. Cheap. But weird.