All art is not equal

PC folk sometimes claim that all culture is equal. A rap song about beating up ones girlfriend is morally equal to a symphony. They say this (I think) because they believe all people are equal and they have given up on the idea that cultures and nations can advance.
The strange thing about this 'equality' is that it is implicitly racist. Nobody should criticise the barbarity of radical Islam (they say) because liberals have decided Arabs are not capable of democracy or reasoned debate.