I'd argue that it isn't. I'm not a legal expert, but it seems that it could be argued that direct messages are implied private conversations, and that laws around recording audio implies that private spoken conversations can occur in offices. The fact that direct messages leave a history is merely a side effect that does not suggest that they are any less private than a spoken conversation behind closed doors.
However, I would need to know the actual intent of such laws, which I don't. Let's say that the intent is to allow for private conversations, then that premise also suggests that messages between two individuals(as opposed to ones in a channel) are only intended to be read by those two participants, hence a conversation that is private. Nobody sends direct messages with the intent that they be read by people besides the recipient.
Why would you need a crude one-sided copy and paste? A password, a cookie, or even an API token, can already provide as much information to authorities as would be provided by that of Slack team admins. There is no need for anyone besides the messaging participants and authorities to see someone's DM history, either technically or philosophically.
Well, as you said, you aren't an expert on this and apparently haven't ever been briefed by your company's legal team, and presumably have never been in charge of compliance. So your arguments about how the law works are in this situation pretty useless.
Jumping away from that angle though, there's still a lot of issues with what you are presenting. For one thing you keep referring to "authorities" without defining who those authorities are. If you're referring to the company IT, HR, and Compliance officers then it seems like you agree with us that the information should be available to those people. However, since that would be a bit odd with the rest of the context you're speaking about I'm going to assume you mean authorities in some sort of government or law enforcement sense.
The thing is that authorities rarely get involved in most of the cases where this information is needed. Sexual Harassment is not a criminal offense, it's a civil one- people don't go to jail for it, they lose money from it. Outside of taking reports these types of things are rarely investigated by authorities in that sense, and there are remarkably different burdens of proof for each of those. Most companies (and individuals I would imagine) also don't want to make a legal issue out of work ones if they can help it, which means it is often in everyone's best interest to handle certain types of problems in house.
Now, as for the philosophy aspect of things, as long as companies are responsible for managing their own trade secrets, sexual harassment complaints, and security in general then all company property (which includes conversations on company servers and services) are open to that company. This is why I do not sign up for company phone plans (except when I want a separate company phone and phone number), and why my work computer does not have my personal accounts on it.
I simply would have hoped that Slack wouldn't give too much control to employers when there are already viable ways of providing message history without resorting to copy-paste. It makes for a lousier product, and it would have prevented me from having candid conversations that involved no company secrets or harassment of any kind.
However, I would need to know the actual intent of such laws, which I don't. Let's say that the intent is to allow for private conversations, then that premise also suggests that messages between two individuals(as opposed to ones in a channel) are only intended to be read by those two participants, hence a conversation that is private. Nobody sends direct messages with the intent that they be read by people besides the recipient.
Why would you need a crude one-sided copy and paste? A password, a cookie, or even an API token, can already provide as much information to authorities as would be provided by that of Slack team admins. There is no need for anyone besides the messaging participants and authorities to see someone's DM history, either technically or philosophically.