So they left the island, why?
Zombies are invading the island, they all leave except for the scientist who thinks it's a good idea to feed his own flesh to his zombie daughter, and suddenly the zombie infestation stops? They should have at least kept the zombies as an ever present threat instead of just having a couple of them locked away in cells.
Or hey, it's a prison, lock it down and stay inside as much as possible.
Yeah, I'm asking for logic from a syfy movie. Oops.