You cannot escape the fact that dressing up a sports franchise in Native American symbols and cultural iconography is racist by pointing out that there are teams called the Celtics and Patriots.
But neither you nor Law, in his article, have established this as a "fact." It has been presented as an opinion and, in Law's case, he takes it for granted but his resulting argument (the teams should change their names because they are all racist) doesn't hold water because his position is fallacious: you can't prove an argument by citing your own unproven premise as support.
Now Matt, if you are saying that, yes, any and all use of Indian-inspired names or iconography (whether historically an actual slur like "redskin" or something neutral -- or even complimentary -- like "brave") is racist simply for existing in that it is a form of "cultural appropriation," then I'm saying logically you must agree that the "cultural appropriation" evidenced by the "Celtics" name and iconography is also racism.
But to me, "Boston Celtics" is not racist just like "Atlanta Braves" is not. "Boston Micks" would be racist, in the same way it is argued that "Washington Redskins" is.