Leaking of business conversations can have serious implications on many areas from financial to legal. If an employee leaves the company how that will be handled.
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.
From the Slack TOS:
>Your acceptance of this TOS gives us the permission to do so and grants us any such rights necessary to provide the service to you, only for the purpose of providing the service (and for no other purpose).
They _could_ be breaking the TOS (and thus the contract between you and the them), but I doubt it.