To many who don't frequent liberal conferences, this headline may sound like a step beyond Animal Farm, with "some lives are more equal than others" too feeble a way of saying what these liberals are really thinking.
I think, though, what's going on is a failure of rhetoric. The words "Black lives matter" are being spoken as a slogan to invoke the problem of black citizens dying at the hands of white police, but they are being heard as an assertion in a free-flowing conversation about race in the United States.
To put it another way, what's being said is "#BlackLivesMatter" and what's being heard is "Resolved: Black lives matter."
As a proposition, "Black lives matter" is unassailable. This very unassailability, though, makes its emphasis in (what is taken to be) a free-flowing conversation about race suspicious. Why press this uncontested point? Is the implication that I am a racist? Is it that black lives matter more?
Hence the reflexive response that got Martin O'Malley booed: "All lives matter." It is equally unassailable, and it counters both worrisome implications.
If anything, the rhetorical failure leads to worse things if it happens when the context of deadly force used against black Americans is recognized, particularly if trust and respect between the parties in the discussion are lacking. Then the impulse is to find an unassailable counterargument to confound those who are saying, "Black lives matter." So we hear, "Are you saying cops lives don't matter?" Or, "Why aren't you chanting, 'Black lives matter!' when black Americans are killed by black criminals?"
This sort of thing can be a sound counter-argument -- but it assumes an argument to counter. The safety of police, and the incidence of black-on-black crime, are legitimate problems. But so is the alleged mistreatment of blacks by police. If every time someone tries to allege the mistreatment of blacks by police, others change the subject, then when do we discuss allegations of police mistreatment of blacks?