That's one of many things they do. (Gang Leader for a Day gives great insight into the many role gangs play in their communities [1])

It's also a possible consequence of Nixon's War on Drugs [2]

[1] http://www.amazon.com/Gang-Leader-Day-Sociologist-Streets/dp...

[2] http://en.m.wikipedia.org/wiki/War_on_Drugs

Gang Leader for a Day: A Rogue Sociologist Takes to the Streets

http://www.amazon.com/Gang-Leader-Day-Sociologist-Streets/dp...

This is about https://en.wikipedia.org/wiki/Robert_Taylor_Homes. The author contributed to the drug dealer economics portion of _Freakonomics_.

Additionally, although many street dealers operate in areas you'd probably associate with being high-crime, the drug gangs are pretty aware that customers need some sense of safety when buying product and don't have any more reason to see their customers robbed or assaulted than any legit shop owner.

If you're interested in an observed account of a drug gang, there is a book called "Gang Leader for a Day" where the author spent a lot of time in a housing project, with the guy who ran that faction of the gang. It has its flaws but it was an interesting read:

http://www.amazon.com/Gang-Leader-Day-Sudhir-Venkatesh-ebook...

Yes yakuza are criminals, but the article also highlights that like many subversive organizations they also provide a number of community functions typically provided by the police (e.g. the article talks about how Yakuza communities tend to have lower crime rates; safety tends to be a service provided by the police). Terrorist organizations often do the same; Hezbollah has been a huge provider of social services in Lebanon, running schools and hospitals: http://en.wikipedia.org/wiki/Hezbollah_social_services

Hernando de Soto talks about how the Shining Path provided social services to build legitimacy in his incredible book, "The Other Path: The Invisible Revolution in the Third World": http://www.amazon.com/The-Other-Path-Invisible-Revolution/dp...

Sudhir Venkatesh also talks in his book about how drug dealers in Chicago provide similar services, in order to be not be rejected by communities inside of public housing projects. http://www.amazon.com/Gang-Leader-Day-Sociologist-Streets/dp...

Yakuza - like drug dealers and terrorists - are capable of great violence, but the photographer's point was that within the yakuza community that he experienced, violence is seen as a last resort. And that community activities that provide legitimacy is an important tool as well, in order to be accepted by the local community and also as a recruitment tool. This is definitely a common theme in the literature and my personal experience as well.

This reminded me of Sudhir Venkatesh's book "Gang Leader For A Day", parts of which were described in Freakonomics. Some of his findings included an organized corporate structure of a big gang, a gang member with an econ degree who wanted to put his education to use, etc. Pretty interesting read.

http://www.amazon.com/Gang-Leader-Day-Sociologist-Streets/dp...

Although its not really in the same vein as hacker news, I really enjoyed Gang Leader for a Day: http://www.amazon.com/Gang-Leader-Day-Sociologist-Streets/dp...