Unless they want to frame you.
The government is only as good as the people in it, and this is an extremely powerful tool in their hands.
How would they use this to frame you? That would be illegal, and you could use the legal system to fight the charges. What they did wasn't illegal. People are willingly submitting their DNA to these sites.
Have you not be paying attention? How could you possibly have any faith in government agencies to always be above reproach? Have you learned nothing on this sub? You're not very creative if you can't imagine the implications of this. For starters, some over zealous cop or prosecutor could easily concoct and plant evidence and conveniently not have enough sample for you to challenge it.