The reason why politics is traditionally frowned upon in the workplace is to avoid toxic peer pressure and the resulting uniformity. It is illegal to discriminate against employees for their political opinions, religion, sexual orientation, race, ethnic background, etc. Allowing or even encouraging open political debates in the workplace jeopardizes it.
Seems more likely it's so that employers can prevent their employees from discussing their working conditions with each other and enforce _more_ uniformity by stifling demands for better treatment.
The previous comment confuses Bullying with Political discussion. Though it is true bullies can participate in political discussions a bully is typically a bully in all areas.