Most people do not have regular contact with Native Americans. Therefore, their understanding of Native American culture and customs comes from the way the people are depicted in popular culture. Mascots depict Native Americans as foolish, war-like and aggressive people. This allows harmful stereotypes to propagate and causes a loss of empathy for Native American people.
Seeing Native American mascots can lead to the spread of misinformation and negative stereotypes surrounding the way Native Americans look, behave, communicate and interact with the world at large. Alternatively, it can give the impression that they have been relegated to warriors of the past, like Vikings, gladiators and knights, neither of which are positive outcomes. Both of these are damaging because they are ultimately dehumanizing and can result in a loss of empathy for Native Americans among sectors of the population whose only contact with Native culture has come from sports teams. A 2016 study found that people who live in the cities of professional sports teams that have adopted Native American mascots and imagery in their branding, subconsciously believed that Native Americans were more war-like and aggressive. The study found that residents of Cleveland, home to the Indians baseball team, were more likely to believe that Native Americans held war-like traits than residents of Miami, Detroit and Atlanta, the control cities.  If Native American mascots are allegedly displaying positive attributes, why would people from these cities have a more negative attitude towards Native Americans?
If anything, these stereotypes are positive. Courage, bravery and strength are all positive traits and are ultimately the reason why the Native American warrior figures were selected as mascots for many professional sports teams. The propagation of positive stereotypes is not harmful. It earns Native Americans respect and honour and fosters a positive image of them as wise, brave and deeply spiritual people.
[P1] Mascots promote negative stereotypes of Native American people. [P2] Therefore, they are offensive.
Rejecting the premises
[Rejecting P1] The sentiments of Native American people present in sports are almost entirely positive. [Rejecting P2] Therefore, they are not offensive.