Why do black lives matter? Because: All lives matter.
That is a non-racist perspective, and it was attacked viciously because it undercuts a propaganda effort.
I don't think that's an honest portrayal of the actual actions that took place or their motivations. In the context of disproportionate black deaths, mischaracterizing focusing on that specific issue:
1) does not inspire any positive change itself,
2) prevents positive change by demonizing those who are bringing awareness to and proposing solutions to the problem.
Rarely do you see the "all lives matter" crowd calling for increased police accountability; in fact, given their close association with Blue Lives Matter, I believe those who throw the slogan around tend to fight for quite the opposite.