How does not discussing the shortcomings of historical figures affect positively (according to textbook authors) and negitivly (according to loewen)?

A traditional white euro-centric historical text book was not interested in facts portraying founding Europeans in a negative light. Pointing that Columbus was largely responsible for a genocide of indigenous peoples was largely avoided. The story of say Pocahontas was best left to the likes of Disney rather than real history. Loewen points out a systematic racial exclusion and bigotry when it comes to history accepted and taught in the United States.