The point is that cities don't have monitored borders, and so they have a hard time regulating the influx of guns. A country-wide ban would actually stand a chance of working.
Edit: here's the article I mentioned: http://www.nytimes.com/2013/01/30/us/strict-chicago-gun-laws...
I haven't heard this but it certainly sounds plausible.
To me, it says one of a few things are happening: (A) The people buying the guns are not criminals at the time of purchase, and shouldn't be prohibited from buying them in the first. Whether they go on to commit a crime with the gun or the gun is stolen and then used in a crime is mostly irrelevant; (B) The people buying the guns are criminals at the time of purchase, and the NICS check didn't alert the owner properly. This would be more a failure of the FBI than anyone else; (C) Least likely, the people buying the guns are criminals at the time of purchase and the owner of the shop knew this and either did not perform the NICS check or performed it with fraudulent data.
I do believe a large majority of Americans would be fine with high gun ownership levels, if only there was a safe, healthy, well informed and responsible gun ownership culture to go with it. Many gun owners do meet that description, but far too many don't and any attempts to rectify that or even provide evidence based advice are blocked at every turn.
It's almost as if the mere act of owning a firearm has little bearing on one's overall safety.