The fact that most of society thinks it's much worse to say mean things about black people than to say mean things about white people is not Twitter's fault.
Does Twitter allow it because "most" of society believes it to be true... or does "most" of our perception of society believe it because Twitter allows it?
True or not, we mustn't let a potentially minority-held opinion overtake the public sphere because the next generation will most certainly believe it because that's all they'll know. That is why absolute free debate is crucial and feelings should have no part in it.