Hospitals can't ban their patients from criticizing them (even if they identify themselves as a patient).
Airlines can't ban their passengers from criticizing them (even if they identify themselves as a passenger).
The general rule is that if an individual is in an economic relationship with a large institution, it is important for them to be able to comment publicly on that institution. This is needed to address they myriad, well known ways that institutions can do stupid or even evil things. It is especially important when there is a huge power imbalance that tends to perpetuate sick institutional behaviours.
And so: Companies shouldn't be able to ban their employees from criticizing them (even if they identify themselves as an employee).