In the Netherlands, in the early 30's we had a census. All the good jewish citizens of the good kingdom of the Netherlands filled in their religion. Because, why shouldn't they? Fast forward a couple of years, and those detailed census results are really handy for the occupying nazis.
During WW II, 95% of the jewish in the Netherlands were killed. Compare this with a country that does not have a central register of it's citizens (France), where "only" 25% of the jewish were killed.
Also, when you give up your DNA, you're not just giving it up for you. You're giving it up for your family.