As was said before, it's a 100% Amero-centric view of racism. A lot of these Tumblr social justice warriors are young Americans who haven't personally witnessed any racism apart from white people vs. non-white people. They haven't heard about Irish oppression outside of history books, but they probably know at least one black person who remembers segregation and modern American history classes are filled to the gills with information on the stuff we put Native Americans through. As far as they're concerned, it's Scott Whitey vs. The World.
I'd be surprised if they've even heard of the ethnic cleansing and discrimination done among Asians, Africans, and Middle Easterners, to say nothing of knowing the discrimination that Sami go through regardless of their skin color.