If these Turban Top Cops can wear Turbans, why can't I display my Ten Commandments plaque in my Federal Government office or wear my Republican Pachyderm pen on my shirt?
Only if it's a requirement of your religion. It's not about free expression. A cop on duty or in uniform does not have free expression. It's about honoring religious requirements.
It can go too far, but this does not. Even if the Sikh turban reminds you of Osama's get-up. It's really not the same design.
Sikhs are in general good citizens, and policeman is the sort of job that traditionally fits in with their culture, believe it or not.
Great Britain had Sikh policemen in turban way before the age of PC, and Sikh soldiers in turban even before that.
Diversity is only bad when made a moral absolute, trumping all else.