They have a cell phone on them because they have WiFi in their house, because they have Internet, and they have notifications turned on to their local emergency servers. Since Twitter has a push notifications and a shorter refresh timer, you get messages more regularly that you’ll actually read.

Compare that to email, which you’d get hundreds of messages a day and only care about 3 of them, so you’re not constantly checking.

Edit: dude you live in Australia. Do you know how people in the Bush get their emergency updates that they need to respond to urgently? I bet at least some of them use Twitter or something similar to it. They're not huddled around their weather radio all day (and that doesn't cover everything). Because I had family in rural California and that is exactly how they got theirs. Twitter and Facebook.

There are places that don’t … have … sufficient cell service, as a starting point.