Your comment explains the hate.

There is no such thing as history without a political philosophy. Herodotus had one, Thucydides had one and Zinn has one. This is all fine and good. The issue is when people come along and say "He tells the truth, a truth not often told".

It's not the truth. It's a contribution. In the medium-to-long run, parts of Zinn's writings will be accepted and other parts will be rejected. In fact, this has already happened. The problem is people selling Zinn as "the last word," as if we can all stop thinking about history now because Zinn finished the job. In reality, history is a never-ending process. Historical theses can be more right than others but none of them are "the truth".