First of all, housing discrimination on the basis of race, religion, and probably sexual orientation by this time, is illegal in California. So the ad broke the law for starters.
Second, the ad was placed in a public forum, so like anything so widely announced is subject to commentary and debate.
Third, we are in an era of Black Lives Matter where individuals and groups maintain that whites institutionally discriminate against “people of color.” If this same aggrieved group wishes to engage in its own racism, then it's rational and understandable that public commentary will follow.
So yes, it's everybody’s business.
Actually, you, SoCalPubbie, are the one who is wrong.
Laws against housing discrimination are applicable to landlords.
Individuals have every right to choose a compatible roommate.
At least for now.