That's not what "all" means. It's turning the name of the movement by implying it means "only" Black Lives Matter. So All Lives Matter is saying that not only 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.
It does happen to black people somewhat more often, per capita, but 50% of people killed by police are white.
Predicting the likely response to this, of course, we get into the (surprisingly hard to accurately work out) question of what proportion of crimes are committed by people of various races, and to what extent that figure is manipulated by conscious / subconscious bias on the part of law enforcement. (It's a swamp, and I don't feel like arguing about it tonight, I just wanted to acknowledge it's complicated.)
Edit to acknowledge: yes, it's more like 60% white if you're splitting out Hispanic-origin into its own category. Still, disproportionate.
If the main problem is (2), but society mobilizes to solve (1), thigs won't get better.
If we solved (1), everyone would be equally likely to suffer from police brutality. This would make us more likely to then go ahead and fix (2), because suddenly it would no longer be something which mostly happens to long-downtrodden minority groups.
Mind, if we fix (2) first, it's somewhat plausible that we might continue in a situation where minority groups are disproportionately targeted by the police, and we just go "eh, it's not that bad, we're not shooting them all the time any more".
I find this argument (the one you are referring to) to be spurious. We don't calculate the number of rapes committed by men and there is no subsequent societal bias against men as criminal by default (or suspect by default). Not to mention the mindset leads to a feedback loop: "Of the bugs found during team code reviews, 50% were in Bob's code" will lead to everyone going through Bob's code with a fine tooth comb - and finding yet more bugs, while Stacy's code never gets a second glance regardless of its quality[1].
1. https://www.washingtonpost.com/news/wonk/wp/2013/06/04/the-b...
The thing is most violent crime is commited by blacks even though they are about 13% of the population. So please don't act like evil cops just hunt them down for fun, that's the biggest gripe with BLM. They act like it's 1900 when that's just BS