What people are intentionally refusing to acknowledge when they say "all lives matter" is that certain lives are under more threat than others right now, and have been throughout our nation's entire history. It's the same type of racially charged cop-out as, "the Civil War was fought over states' rights." Just because a number of historical and modern events are uncomfortable to discuss from the perspective of privilege doesn't mean the answer is to stick your head in the sand and pretend they never happened.
Additionally, a certain amount of nihilism always comes attached with alt-right ideologies, so what a lot of people really mean when they say, "all lives matter" is, "no lives matter." After all, there is no actual organization by that name, "ALM" is nowhere to be found when police brutalize or murder anybody of any race/ethnicity. In other words, it's an endorsement of the status quo, and the only group that benefits from such an endorsement is the police. I say if you're gonna lick boot, at least be honest and straightforward about your intention to do so.