A lot of this art comes from groups that are really active in and around Oaxaca City. As the link states, much of this protest art stems from the violence and social unrest that took place starting around 2006. There was a huge reaction against the government, specifically governor URO (Ulises Ruiz Ortiz), who many claimed took office under fraudulent circumstances in 2004. In 2006 there were violent clashes between the government and Section 22 of the SNTE (teachers union), in what initially started out as a protest about teacher’s salaries. The teachers protested, and the government reacted by trying to forcefully remove them. Other groups joined the teachers in protest, resulting in a pretty substantial political conflict. Of course, it was anything but a clear-cut political issue. When I was there during the summers of 2007-2008, the streets were filled with all kinds of social and political art/graffiti, from the kind of work pictured above to murals, stencils, and spray painted political epithets and messages. There was a good amount of media coverage about these events. Check out the March 2007 issue of Anthropology News for some short articles by Ron Waterbury, Lynn Stephen, and Deborah Poole. I think that Kristin Norget may have written about some of this as well in a 2009 article.