If Native American communities were genuinely offended by the use of their imagery in sports teams’ names and mascots then it would be offensive. But most of the people expressing offence are not Native American. In fact, when polled, nine out of ten Native Americans have no issue with their depiction as mascots or the use of their names. 
Just because other demographics say they should be offended doesn’t make the act offensive. If the actual community under “attack” isn’t offended, then the act isn’t offensive.
Many high schools located on Native American reservations actually use the term ‘Redskin’ in the names of their sports teams. They also use Native American figures as mascots. If they are embracing the term and are not offended, then it is clearly not a problem.