Some people have undoubtedly co-opted and perverted the "Black lives matter" movement for their own ends, but the slogan is absolutely not exclusive in the way people who reply "all lives matter" back. Of course they do. The recent death of yet more policemen is shocking, tragic and wholly unwarranted as a reaction. But just to be clear, if someone says "Black Lives Matter", they [should] mean it in the same way as a slogan to "save our rainforests". Saying that wouldn't mean "I don't care about any other type of forest"; merely that, at the moment, the rainforest is particularly under threat currently and so merits extra attention. Black people in the US are, or appear to be, particularly in danger of police discrimination ending in death (by about 2 times proportionally, compared to White people). Hence, a "Black Lives Matter" slogan is wholly reasonable. It doesn't mean not caring about any other type of life. There's no reason to respond "All Lives Matter" as if we didn't know this already. It misses the point.