They do, but "All Lives Matter" is a purposeful and pernicious misunderstanding of what "Black Lives Matter" means, which is something along the lines of (quoted from a law professor[1], and I think it could be a little better-turned but you'll get the idea):

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