When you think about it, it makes perfect sense. In case Dharma loses somebody, if they die, go crazy, leave, anything, they'll stop pressing the button, right? So, if the button isn't pushed, the...whatever it is, goes off, the electromagnetic field. When it goes off, it drags down a plane or maybe a boat from the shore and viola! New button-pushers!
But then, of course, you have to wonder why that didn't happen when the button was pushed by 'Henry Gale' when Locke was trapped in the hatch. Although, since he seems to be pretty high up in ranks with The Others, it's possible that he's in on 'it' whatever it may be, and he knows another code, which is why the food dropped.