It doesn't work that way in English. If you call it out it's because you feel there is something distinct about it that is different from the other members of its group.
Saying "Black lives matter" implicitly says "and others don't", and even if you won't agree to that, at a bare minimum it says "black lives matter more than other lives".
It doesn't help that this slogan arose in a context of defending a black life that was trying to take another life, or at least harm another life. By defending such a person, as they have done, they are very clearly saying "that black life matters more than other life".
Context matters. Every time they use that saying it reminds people that these activists are defending attempted murderers.