They probably just want the DNA to verify your identity. Except how would they do that unless they already had a sample on file? What is most disturbing though is that DNA is exclusionary, not inclusive. If the DNA doesn’t match you are clear, however if it does, there is a significant statistically likelihood you’re still innocent if the evidence is based on DNA alone. Other forms of evidence need to be provided to illustrate your connection to the crime.
Many probably wouldn’t care, however this information is becoming more and more sensitive due to genetic conditions. There is still the question of how exactly it’s going to be used, and if they’re going to try and skirt the 4th Amendment again. Never the less, the rapid acquisition of power by the DHS is very concerning, and they’re only gaining more.