I imagine some people worry how they would deal with getting lost or being injured. It’s a fair point because so much now assumes you will have a smartphone with you at all times.
That’s why it’s nice having somewhere that doesn’t assume that.
If you get lost on this 1km island, just pull out your phone from your backpack and call for help. No one will stop you.
So what's the point of the sticker? can't put your phone in your backpack?
Presumably the point was getting on the news to attract a few more tourists.
Encourage people to use the phone less. And probably also as a promotion for the island.