I don't think you appreciate the scale of how many headaches this can create for a large company. Previously, A says something mean about B at the watercooler, even if somebody overhears it's all he says, she says. With Slack (or any other group chat/IM system), if A says the same thing, it's broadcast to everybody else on the channel and anybody can take out of context screenshots and run squealing to HR/EEOC/lawyers/whoever. And it's all accessible to anybody with a subpoena on a fishing expedition as well.
And you can't just fix the company or the people: beyond a certain scale, no matter how good your hiring process and training, no matter how awesome your company culture and benefits, you'll get permanently unhappy people, shit-stirring trolls, wackos with extreme views and all the other weird creatures that populate any Internet forum. So eventually you have to converge on reserving chat only for strictly work-related comms and training people to take anything with the mildest whiff of controversy to talking in person/over video.