Serge - You are correct sir. The individual who said "That's racist!" has every right to express his political opinion, but NOT when he is on the job. Having spent part of my life working in the public sector, it was constantly emphasized that when dealing with the public, you did not express personal political views - and rightfully so.
As to my opinion that the employee needs to be disciplined. I am certainly not advocating firing or even suspension. A "verbal reprimand" should be sufficient to remind that employee to not engage in political discussions when on the job. It is unprofessional.