Because literally every company in existence would think that employees publicly airing their grievances about their employer on Twitter is strictly less of a p

Because literally every company in existence would think that employees publicly airing their grievances about their employer on Twitter is strictly less of a problem than on the company's internal channels. If you think people should be legally protected for anything they say publicly on Twitter, then you should also believe they should be legally protected for anything they say in company internal channels. Anything else is incoherent.