So anti-racist people who happen to have low melanin levels should feel more of the despicable nature of racism... Why?

For education purposes. This thing didn't hurt anyone physically (thank god) so why not take it as an opportunity to get a different perspective and viewpoint than the typical suburban view?

Is revenge directed against the innocent justice to you?

Its not justice. Its just frustration directed at innocent people irrationally. Kind of like how people have anti-muslim sentiment and chastise innocent law abiding Muslims and sikhs(who are not even muslims) for actions of few. Or a "concerned citizen" calling police after spotting a suspicious male who gets followed and possibly detained by the police for the crime of being black. Just like how the white mother clutches her kids children closely towards her after spotting a black person in the street. These things are irrational. And black people aren't immune to it. They see a white guy in a black lives matter event and feel irrational emotions surging and direct it at innocent white people. I am all ears to solving this irrational attitudes.

Gives you some kind of emotional gratification?

Unfortunately for those people, yes. It does give them gratification. How should I put it....Do you know that feeling when you play a video game for a long time and lose and you have a rage quit moment and you lash out at your mom for talking to you? You didn't exactly got mad at your mom specifically, you just had a temporary moment of anger. Its kind of like that.

And all it does is give the actual racists more ammunition to justify their poorly thought out reasoning, undermining the whole point of such a rally.

Well, racists take every available opportunity to confirm their worldview.So its not like they are suddenly going to change their view just because they weren't being all inclusive.