Well, I think an employer has the right to limit non-work related activities during work hours. But you are right, they should, by the same token, limit pro-Obama talk too.
This employer is the county government.
I'm sure that there are plenty of "progressives" that, should somebody post the 10 commandments in the office, the First Amendment absolutely applies in that workplace. (Forget the fact that plain wording of the amendment applies to the federal congress making laws, and does not contain the words "separation of church and state").
So, the very same amendment should be applicable to the government employer when it comes to freedom of speech, right?