Also, there’s the causality question. Here’s a quote from an article on the study:

Criminologist Gary Kleck of Florida State University said the Penn results can be explained by the fact that people who are at risk of being shot are also more likely to buy or carry guns. Such people might have dangerous jobs or belong to a street gang or be involved in the illicit drug trade, for example.

In an e-mail, Kleck explained his view with an analogy. “It is precisely as if medical researchers found that insulin use is more common among persons who suffer from diabetes than among those who are not diabetic (something that is most assuredly true), and concluded that insulin use raises one’s risk of diabetes.”