For those still doubting, this is not a hypothetical case.
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.
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.