Well, they were following Michael, who was heading straight north. If the Swan hatch is at the southern end of the island, then they were heading straight towards the center… or the northernmost one. Or something along that line.
Bear in mind that Bluebeard (as Sawyer calls him) didn’t say the other side of the line was Other territory; he just said it was a line the lostaways weren’t supposed to cross.
I liked another Doper’s suggestion that he didn’t want them to stumble upon Club Med.
The producers early on said that moving the plane back and forth between scenes in Hawaii was getting time consuming and expensive. They invented the “tide has inexplicable risen and we must move closer inland” event merely to save on production costs and timing.
Dammit, that’s no fun at all. 
It’s odd how often this comes up, and I’ve ended up defending my position far more often than my level of dedication to it justifies, but I continue to maintain that, despite this perfectly valid external reason, the writers need to include an “in-story” explanation. Jack can’t turn to Locke and say, “Oh, well, the people paying those guy who always follow us around with cameras couldn’t afford to leave that plane there.”
Ooh! I know!
It turns out that Hurley was just moving Sayid’s yard sticks further out to sea, because he was using them to anchor his crab traps and that’s where the crabs were.
That was Episode 42, by the way – so I guess that settles it.
If they have to stretch their original intent for the story in order to explain this, then they’re changing their story, possibly worsening it, just to silence a very minor nitpick.
Yeah, it’s too bad that they didn’t think of some reason to make them move that didn’t have such huge (unintended) implications, though. Sand fleas, or something.