You're missing the point. You don't actively monitor it, you record it so that later you can go back and review those conversations in the event you are required to by law. I don't know where you've worked, but this standard in any sizable company, or any involved in particular industries, and is not that difficult to arrange. There are specific legal requirements to keep records of certain types around for 2, 5, 10+ years, whether it's email, chat, file servers, etc. And yes, that includes Slack.

Why do you think Slack is any different than the systems that we have in place already? What makes Slack any different from email? Answer: nothing.

And if Slack didn't do this, they'd eventually find themselves filtered out of nearly every corporate network due to the inherent legal risk.