Do you not see the difference between beliefs espoused by a group as a whole, and those espoused by singular members within a group?Sounds like a double standard there.
It's a diverse group, & not all want things positive.
/snip/
But you'd attribute these beliefs to all who march with them.
You haven't supported this claim
If you march with the KKK, you know you are marching with a group who believes that white people are better than all the rest and that other races should be exterminated, removed, or subservient.
If you march with BLM, you do not march with a group that espouses cop murder. You may inadvertently march with individuals who hold such beliefs but such interactions are unavoidable unless you know the beliefs of every single person you interact with.
It's unreasonable to hold people accountable for things they don't know (like the awful beliefs of individuals), but it is reasonable to hold people accountable for the things they do know (like the awful beliefs propagated by a group).