If one truly believed "all lives matter" then one would not object to the statement "black lives matter".
O'Malley was eviscerated for that statement, even though he clearly agreed with the sentiment that black lives matter.
I can understand the concern that some have that turning "Black Lives Matter" into "All Lives Matter" could conceal the reality that people with darker skin have been disproportionately impacted by our Justice system, but I also agree with the contention that overreaching and overreacting to a statement that seems completely reasonable to people not deeply aware of the discussion can ultimately have the opposite effect and serve as fuel to movements like the alt right.