Many companies want to have free expression for the most part, but discussing politics at work can be challenging and divisive. Like religion and cultural discussion, politics involve deep personally held views, which many are unable to discuss in a manner respectful to others.
Unless one works at News Corp, Time Warners, or other news outlets, political discussions are outside of what one is paid to do. Therefore, limiting these discussions and potential arguments can help maintain productivity. Many companies limit a variety of discussions in the workplace, especially thoughts and feelings about management efficacy. Political discussions may help reduce tensions in many already challenging work environments.
If workers want to discuss political issues with colleagues, they can always do so during happy hours or offsite lunches. That is a fair exceptions. Therefore, people can ensure conversations are between parties willing to participate in such discussions.