As such, employers should strongly prefer that such communication happens via internal channels. It is the smart thing to do, unless you believe that employees would just stop discussing these things if you banned them from internal IM. But of course, this is Apple.
Even in the context of companies that often have slack channels where people discuss all sorts of topics ... it's one I would expect many people would want away from the prying eyes of the folks who chose to pay them X.
I wonder if there is some expectation / hope / motivation the the employer would see the topic and somehow respond? A sort of slack channel as a psudo-protest / online activism?