Below this is an example of political satire. One of the freedoms we enjoy in America is the freedom of speech. Some people are very blunt and just say whatever they want to. Political satire is a clever way to criticize the government without being totally rude. And many times, a message is passed around much more if it makes people laugh.
The comic below from just before the 2012 presidential election basically says, "I can't stand either candidate for president, and their supporters are dumb." That comes across as angry, and doesn't make you care much.
But with the visual joke of a zombie looking for brains and passing up supporters of the presidential candidates, we laugh a little. The message becomes one we can connect with because we figured it out and it made us feel good.