As a turban wearing Sikh.......
Can't speak as a member of the police or services I'm just a keyboard warrior ...
I believe rules and regs state that you have to have the correct protective measures. As has been stated earlier lots of Sikhs are happy to take the risk and let God look after them (as they are want to do on motor cycles in the Uk and in the armed forces for other countries) Sikhs have a martial history and have served in her majesty's forces whilst wearing turbans in several campaigns without whinging and moaning about it.
The sensible course of action would be to let them take the risk - they know what a bullet is capable of doing and it is there choice to forego protection. The nanny state would not be happy with that however, and therefore puts up barriers.
Equally those Sikh's unwilling to go into that job without protection should not expect special treatment.
Rather than explain the why's and wherefore's of Turban wearing I'l just link : http://www.sikhiwiki.org/index.php?title=Turban