I think there's a problem is there are multiple definitions of racism going around that's not helping people understand each other.

One of them means any kind of racial judgement or prejudice. Some people take this to the extreme where even acknowledging that race exists in society is racist. Like when people got bothered just by the phrase Black Lives Matter before it became associated with riots. Or when someone lowers their voice to say someone is black.

Another necessitates that power be a part of the definition, and folks will say racism = prejudice + power. White people as a group have historically held power over minorities. So while minorities can be prejudice and make racially charged attacks on white people, ultimately white people have the power. This is why some people say minorities can't be racist.

So in these situations the protestor with the megaphone is thinking in terms of the latter definition in that the white guy telling black people how to protest for their cause.

Even though I don't think he's wrong, they don't want white people telling them how to protest because white people have been telling them what to do for 400 years.

Either way, antagonizing some guy on his porch who's sympathetic to your cause doesn't seem like a good look to me.