In this way they would hope to contain the virus without innoculating the general population.
If this is their theory, giving smallpox vaccine to only citizens within the ring and not to noncitizens within the ring would tend to defeat the purpose of attempting to halt the spread of the disease by vaccinating everyone within the ring It would, of course, tend to kill off most of the unvaccinated noncitizens, but it would also lead to a more widespread contagion area.
Of course, I could be wrong.