Ben is the most interesting character on Lost because of his intimate knowledge of both on island and off island, and the reasons for both parties (hostiles and Dharma) for wanting to have control over the island. Ben knows the island is crucial to the existence of men everywhere, but doesn't yet quite know why. with that being said, he doesn't quite understand what the Swan's purpose is, but he may have felt nervous about halting it's operation. He could not explain to one of the Hostiles why they should keep pushing the button, so he kept the ex-Dharma dude in there to do the work for him.
Now, the quarantine was printed on the door to keep old dharma people from leaving and getting 'mind attacked' by smokey or Jacob, but once ben realized that both could be under ben's control, he bought chemical suits to Kelvin telling him that he must remain inside or out with a gas mask to prevent a gas attack. He did this to keep Kelvin loyal to Dharma, but really, to ben himself.

Geez, it's so obvious....LOL