It’s not that you are wrong when you say “All lives matter”, heck I might find my self saying exactly this when I speak out against death penalty or bomb manufacturers.
But when you say this as a response to BLM you are being an asshole. You are saying that I can’t emphasize the importance of black people’s right to life in a society that seemingly to me treats them as lesser.
No, when one says it in response to BLM, they’re neither being an asshole nor or they saying you can’t emphasize anything. You can say whatever you want obviously. When people say it, they mean it literally, and they believe that the divisive rhetoric is harmful. Which seems to be supported by the evidence.
The slogan itself was born of a lie and a false narrative, which they continue to perpetuate. There’s no reason to continue allowing its use as a political weapon.