The hatches with "quarantine" marked inside are part of the psychological experiment(s) being performed by the DHARMA Initiative staff. Those without the markings are for the use of the staff and not part of the experiment.
Is fake, and is used to make sure that the people inside the hatch don't go outside. This could be because the DHARMA Initiative doesn't want the people inside seeing the rest of the island or exploring and finding more hatches.
The quarantine's function is to keep people inside the stations to protect them against the Hostiles.
Quarantine signage is usually visible from OUTSIDE of the quarantine area in order to prevent people from venturing further. The fact that the quarantine signage is visible from INSIDE the hatches means that it is meant to keep the hatch occupants from venturing outside.
This could be a reference to "The Hitchhiker's Guide to the Galaxy" books by Douglas Adams (which perhaps not coincidentally attribute great importance to the number 42). A character called Wonko the Sane built a house he called "The Outside of the Asylum" when he determined the rest of the world was insane. His home was the only sane place that existed. By extension, the hatch is the only place that is safe from the disease that afflicts the island (or possibly the entire world).
The number coded message on two of the Jigsaw Puzzles may confirm that the quarantine was indeed faked, but offers no clues as to why.
The quarantine may indeed have been faked as part of a psychological experiment on individuals inside the hatch.
The quarantine may also have been faked by Ben and the Others after the Purge, to keep the station inhabitants in their stations, and keep their various experiments running. The Monster would then be a security system meant to enforce the Quarantine, and perhaps deliver the Periodic Food Drops as well.
The quarantine door that fell from the sky after the discharge was not a result of the implosion of the Swan.
Perhaps the quarantine door was actually pulled down from above by the electromagnetic pulse... off of a satellite or GEO-DOME exterior.
The quarantine in the hatch, the hazmat suits and the regular injections taken in the swan are to protect against the gas released from the Tempest.
Some have posited the quarantine may be due to the detonation of the Jughead in the Incident.
The quarantine was just a ruse that was concocted after the Incident, to make certain people believe that a nuclear bomb had been detonated at the site of the Swan.