Black Lives Matter! because they haven't mattered in the U.S. for over 400 years and all lives can not matter until Black Lives Matter.
Why? Because they have been minimized, marginalized, criminalized and policed, literally to death. Modern slavery can be found without chains and whips.
As someone who actually saw Philando Castile, immediately after he was murdered, on FB Live, at the time it was happening, Black Lives Matter is at the very least an organization of community responsibility and accountability to the struggle of Black Lives in America.
Black Lives in America have been marginalized from long before they even stepped foot on this land, and then every since. Much like the Civil Rights struggle, if you are not "in the struggle", no matter your color, you will likely see the movement as the media paints it for most outsiders.
Black Lives Matter is merely an extension of the struggle that has been going on for centuries, dating back before the Underground Railroad. The only difference between them is the ability for modern technology to bring more people together in much less time.
The rioting and looting that some commenters have decided to put their focus on were brought on by many disingenuous actors being opportunistic and were not welcomed behavior by the organizers of those peaceful protests.
And I don't blame them one bit for closing down the highways, in peaceful protest. It is about bringing awareness to EVERYONE, so that people who don't know what's happening but have good in them towards others, will join in the outrage over the centuries long mistreatment of Black Lives in America.