Some companies want subservient, homogenous employees that come in, do work, and can be let go if they do not perform. That's a simple equation.

If you get in somebody who is a star, however minor, that changes the equation, changes the dynamic. Now that person can have more confidence, can have more sway in the decision making. If the company wants to let them go, then they might post a message to their followers, riling them up, creating bad PR for the company. It's no longer a simple equation.

So it all comes down to the insecurities of the company.