No issue. At all.
Yes, it is an issue. If I am approached by someone in a turban, I do not perceive that I am interacting with a police officer.
Like everyone else, Sikhs need to decide if they are a cop or soldier and follow uniform standards.
No problem with Sikhs, but they should wear the same uniform as every other cop.
Sikh officers with a large knife can be intimidating to law breakers : )