The main thesis of Loewen's book is that American history has been taught in a way that glosses over the bad parts of what Americans have done. Loewen argues that American history has been meant not to really teach us about what happened but rather to instill in us the idea that our country is always good.
To Loewen, typical history texts tend to try to ignore the bad parts of US history. They try to tell us that America has always done the right thing. As Loewen says in the illinois.edu link below,
They portray history as a simple-minded morality play.
In that morality play, the US is always on the side of good and we can admire essentially everything about our history.
Loewen is also making a number of other points about the way history is taught and the quality (or lack thereof) of history texts, but his main thesis is that history is made into a simple tale that is meant to show how great the US is.