Because of this graphic:
http://www.nationalgangcenter.gov/Survey-Analysis/Demographics
Now, you take this graphic, combine it with a fair share of racism and generalization: voila, you've got yourself a biased representation of "blacks" as a whole. Face it, people don't like thinking too much, and people LOVE to generalize (or are too stupid not too, either). It's simple from there.
This alone will be enough to convince a good amount of people to blame culture. Though process is very simple. Black kid with a gun commits heinous crime, seemingly for no reason other than personal monetary gain (at least that's what it will be perceived as at first) -> obviously is gang member -> "black/ghetto culture" (or whatever it's called).