They are leaking potentially confidential information to a third party company running the service, in this case Slack. For all you know they could be doing stock trades based on all the insider info they can glean from their chat traffic as their business model.

If the communication includes material insider information, like companies/products they are considering buying, and they are flinging it all over the internet so third parties can read and act on it the SEC can charge them.

If you use cloud services for company communication then you at least need to have provably secure encryption so only the people you want to see the conversation can.

Of course this criticism applies to all the suckers who use Google and Microsoft cloud services for their business.