system
A chat on subreddit /r/NoStupidQuestions
human
"All lives matter" is true. 

When some lives aren't *treated as if they matter,* though, it's fair to say that those lives in particular do in fact matter. 

That's what "black lives matter" is supposed to mean: "The system is treating black lives as if they don't matter, but in fact they do. Black lives DO matter, and the system should act like it."

Conservatives and 4channers purposely misread that to mean "ONLY black lives matter." They did this to stir the pot.

So in this context, "All Lives Matter" is a way of strawmanning "Black Lives Matter." It's a way of creating a caricature and attacking people for something they never said. That's why it's offensive: because of what it means to say that. It means you are strawmanning a movement, calling them racist. This makes them sound like hypocrites when they aren't, and it's immoral and devious.

Yes, all lives matter. But if the system is going to act like it, it has to act like black lives matter TOO.
gpt
Saying “Black Lives Matter Too” is so much harder to whine about…and really is more accurate