My understanding is that the Sikh religion requires the men to serve in their country’s military, if able, to protect their countrymen. It may be part of the same belief system that requires them to carry a knife to protect anyone who needs aid (if the laws in the area don’t permit it, they can wear a tiny one as a symbol within the turban.)
Under a law signed by Harry Truman, American Sikhs were allowed to enlist in the U.S, Army and were given modified grooming standards that allowed them to retain beards, not cut their hair (which is bound up under the turban), and wear a turban in uniform. (I think the law was rescinded about 10 years or so ago.) Before that, I had Sikhs for both my first sergeant and a platoon leader in my army unit, both very good, very squared-away soldiers. It was always kind of interesting to go to a different base and watch the double-takes when people saw soldiers with turbans and beards in U.S. uniforms.