I'm not sure if you're arguing for or against "too".

Because yah, police specifically, and society generally, have been acting as though black lives don't matter. And the slogan "black lives matter" was created to argue against that idea. But it was easily confusing. Hell I was immediately confused the first time I heard it, and actually thought "Well yah. All lives matter. What are they talking about?" It took me a good min or two to understand. But simply adding the "too" immediately clarifies that.

"Black lives matter" isn't wrong. It's just not immediately as clear as it could be.