What is the difference between the situations? All I remember of HipChat's change was the same thing: conversations would only be logged moving forward after a particular date (rather than a switch being flipped).
On Slack, from what they describe, the feature is only available with a certain type of account, you can only export logs in bulk (i.e. it's not something easy to do, it has been made for a specific purpose), and employees will be notified if the feature is enabled/used.