Racism so exists on the left, it is not just the right that engages in racism and homophobia, yes the right is more overtly racist in being anti Black or indeed anti Asian but when you see " Whites not allowed" that is an echo of the past and it is the left that is doing it.
Serious racism exists in America nobody denies that but more subtle forms of racism also exist and an instance like this goes to show America is not as inclusive as we like to believe. Model for pluralism, I think not. To give you an example from Ireland recently a NI football player was given a 10 match ban for sporting a pro IRA t-shirt now traditionally it is the Unionists that are the sectarian bigots.