They could come and go because they could be found; that’s why you had to keep pressing the button, to dissipate the EM energy whatever that causes the timeskips. Here’s my current understanding:
Pre-Dharma:
There’s a funky island that skips through time and space, but doesn’t really do any harm, it’s a natural supernatural thing. There may or may not be original inhabitants from long ago with 4 toes.
By sheer luck, a sailing ship makes it to the island. It crashes, end of story. Maybe they messed with the EM thingy, maybe no.
Somebody does some funky math and figures out that somewhere, something like the island exists. Which is a pretty common method in particle physics, so I can buy it for the island as well.
The Dharma initiative is born to find and study the island.
Dharma:
The DI studies the thingy for quite some time, until . . .
They drill too close to it/disturb it. Oh fuck, now the thing is going to go off all crazy. Or . . .
The thing was going off anyway, so . . .
The build the station to dissipate the energy, thus keeping the island in place, thus allowing them to come and go as they please.
Until the natives go to war with the DI. Ben, being the master manipulator, managed to control the natives as well as bringing in people to keep pushing the button, thus keeping the island and the DI happy at the same time.
Then the thing goes off, and Oceanic crashes. Ben installs someone as quickly as possible to start pushing the button so it doesn’t happen again. For a while he’s on his own and everyone gets a bit ragged, but the DI finds them eventually and transport/supplies continue.
Locke doesn’t push the button, Desmond does something or another that lets the island start naturally skipping again. But the DI’s interference have made the skipping too frequent and dangerous now.
And that’s all I’ve figured out. Corrections/expansions welcome.
H.