Zinn is an ideologue who picks his facts based on the social goal he is trying to achieve (imposition of a Marxist system in the US):

http://hnn.us/articles/1493.html:

“Objectivity is impossible,” Zinn once remarked, “and it is also undesirable. That is, if it were possible it would be undesirable, because if you have any kind of a social aim, if you think history should serve society in some way; should serve the progress of the human race; should serve justice in some way, then it requires that you make your selection on the basis of what you think will advance causes of humanity.”