If 6 black people's homes were on fire, and 4 white people's homes were on fire, wouldn't it make more sense to send firefighters to all the houses?
It should be recognised that having a violent interaction with police in America occurs more often than every other first world country regardless if your black or white.
Moreover, it assumes that the way in which they don't matter equally is based only on racial lines. I'm far from convinced that's completely true. It appears that's some of it, but the fact is that police kill lots of innocent whites, hispanics, and asians as well. That being the case, wouldn't it make more sense to look more broadly at where the disparities are, rather than simply assuming that it's all racial while refusing to entertain any other possibility?
Because of the brutalizing and killing of black people at the hands of the police and the indifference of society in general and the criminal justice system in particular, it is important that we say that Black Lives Matter.
This is, of course, too long to fit on a T-shirt. One might wish that they'd gone with "Black Lives Matter, Too", but that isn't as rhetorically strong and smacks of me-tooism, of begging to be treated like humans rather than asserting humanity. Either way, "All Lives Matter" is at its heart a racist dog-whistle precisely because it attempts to diminish the current, very active threat caused to black communities by the police and the justice systems that should exist to help them (instead of exist to maintain a profoundly racist status quo). If you are not on board with this, I would advise not using the phrase.
[1] - http://imgur.com/a/YkDVQ