The problem with identity politics, the likes of which Kaepernick promotes, is that it plays into the hands of the oligarchy by promoting class-ignorant liberation, further fostering divisions among working class ranks. This is also true of the concept of white privilege, which is more or less the ruling class looking for someone to share the blame with... rich white people hiding behind working class whites, using them as a shield (i.e "we're all in this together"). And if you want to overcome "white supremacy" you have to overcome all whites, essentially putting poor whites on the front lines of a social fight of no benefit to themselves; indeed, while the working classes pelt one another in the streets, the true perpetrators of societal injustice and beneficiaries of privilege virtue signal from their gated white communities.
It's essentially divide and conquer - the white working class is the largest of the working classes and therefore the bourgeoisie is behooved to keep them alienated from other proletarians, lest the workers unite and seize control of social production for themselves. Kaepernick is a product of this social engineering.