What is Howard Zinn's problem with the way historians have reported history in the past?

Howard Zinn wrote an important account of America's past with A People's History of the United States.  He felt compelled to write the book because he believed that history was being taught incorrectly.  Zinn felt that history was being presented from the perspective of the oppressor.  He presents this idea in the first chapter of his book and uses the story of Christopher Columbus as his first evidence.  Zinn believes that the story of history should be presented through the eyes of the oppressed. Zinn felt that presenting Columbus as a heroic explorer and conqueror was a grave injustice to the populations of indigenous people that he exploited and wiped out. Zinn feels that historians deliberately mention the negative aspects of oppressors in passing and focus on the great deeds.