Wait... who is the #alllivesmatter crowd? It seems like the blacklivesmatter crowd is at least talking about it, and upset about it. I can see why Fox News wouldn't want to talk about it, but if anything this only strengthens the argument for police reform, and for those who have generally been demanding it. I'm still not sure who the #alllivesmatter crowd refers to, though. Edit: Nevermind. The #alllivesmatter crowd is pretty much Fox News, so it makes sense. I was thinking this was going after Martin O'Malley et al., which didn't make sense to me at first.
The #AllLivesMatter crowd came out in force on twitter trying to silence the #BlackLivesMatter trend. It's popped up outside of twitter too, and no it wasn't just Fox. Various left-of-Fox presidential candidates have used it as a response too, but some did at least apologize. Also I'm not sure right now if you are understanding the significance of the retort, so if you are (and for anyone else reading this who is scratching their head) I'm going to point to this comment on reddit which explains it well.