It's a common conversation: A person says "Black Lifes Matter", another responds with "All Lifes Matters". The phrase "Black Lives Matter" also has an implicit "too" at the end: it's saying that black lives should also matter. By responding to this by saying "all lives matter" is willfully going back to ignoring the problem. It's a way of dismissing the statement by falsely suggesting that it means "only black lives matter," when that is obviously not the case. And so saying "all lives matter"as a direct response to "black lives matter" is essentially saying that we should just go back to ignoring the problem.