The problems are that DNA is not the unique identifier that "law enforcement" would have you believe. They don't measure everything and record it. That information is larger than an encyclopedia printed out.

So, why are they trying so hard for something that is not particularly true?

They could be looking for human/alien hybrids.
Or maybe they are looking for any Tartarians that got away.

Or maybe they just want to have the ultimate "you did it" database, that they can hold over everyone.

Whatever it is, they are not being truthful in why they are pushing so hard.