Makes some sense to me.
In some cases, as Slack says, there may be a legal mandate to log employee conversations, but in other situations there may be legal restrictions on reading employee-to-employee conversations. That all probably varies by jurisdiction.
And then you have more complicated situations, like companies that use Slack to offer tech support to their customers, or random open-source projects or local volunteer projects using Slack. They might pay for a business license for various features, but it's probably not clear to every member that that would mean whoever set up the Slack account should get to read everyone else's correspondence.
You also want some kind of safety check to make sure that a random IT guy who set up the Slack system at a small company isn't reading through people's DMs and private channels to stalk people or access confidential information.