They're preventing third party messaging apps from running on company devices. It's no different than not allowing someone to run Google Hangouts on their work computer. It's a non-story.
They more than likely have an in-house IM product which is compliant. So company communication is done using company tools.
This whole BYOD craze still has me shaking my head. Why would I want to be connected to work 24/7/365.25? When I leave work I leave work. My cell phone number is on my public contact card if people need to reach me after hours.