Not cutting their hair is a requirement of the religion. No word on how it should be worn. They could ditch the turban and still be observant.
Its not a requirement that they don't cut their hair to call themselves Sikh. Many Sikhs cut their hair and still practice their religion.
You are Indian, so I don't know how you can say that. The very religious people still keep their hair(kesh) and many of the new generation does not.
The same applies to very religious Jews or Christans or Hindus or Muslims. Not all people follow practices that were adopted hundreds or thousands of years ago. Some do.
But they are no less for following or not following something like hair cutting.