Why? All US military personnel are finger printed. If one works at many state or federal agencies they are fingerprinted. If a US citizen is arrested, they are finger printed. Heck, at places like K-Mart, parents can get their children fingerprinted.
Now, they are holding free DNA swabbing of children in many areas of the country. Many states take DNA samples from felons.
In principle, I am against forced fingerprinting and DNA swabbing, unless one is a criminal. However, we have all read so much how the terrorists and those who support them flit and fly from country to country. They come into this country repeatedly. To me, it makes sense, to use an identifier that they can not change at will.