I feel like the mods are like police in nightclub districts, in the sense that the police know that drugs are inevitable in the inner city. So they let people get away with smoking a little bit of marijuana and taking ecstasy into clubs, and then they go for the really serious drugs like heroin and meth. That's what the mods do with mess. They know it's inevitable. So they leave fighting threads open for a little bit (like the Iggy/Azealia ones that are currently going on) just to let everyone get it out of their system.
That's the point. The cause is #BLACKLIVESMATTER for a reason. The cause is NOT #AllLivesMatter. She is NOT supporting THE CAUSE. If she had supported the cause of #BlackLivesMatter this whole thread wouldn't exist right now, but she didn't. She is ATTEMPTING to support the cause by jumping on the bandwagon and tweeting some faux-empathy to make herself feel better about herself, but because she spent two seconds thinking about the cause and zero effort learning about it, she ignorantly diluted the cause to #AllLivesMatter when that is not the issue at all.
This is NOT just about Katy Perry either--all of the people who tried to make this about #AllLivesMatter deserve the same response, but Katy's history of exploiting homophobia, cultural appropriation, and general bandwagoning of social causes she knows nothing about for commercial success makes this tweet not an anomaly of an event but a trademark of her career.