Gun Control & RKBA

...does that not undermine the "imported guns" argument? If not, why not?

Those imported guns may facilitate the crimes committed, but for whatever reason, there are more people who elect to commit those crimes in Camden (for example). The OP's question seems to me to be asking why this is the case. There is a large criminal demand for guns in these areas, and the guns didn't cause that demand. I would assert that if that demand isn't lessened, no (feasible and do-able) gun control measure is likely to make a dent.