I agree with you -- there are real problems with this sort of method. And where does it end? Should they be able to fingerprint everyone in town? Monitor everyone's phone and computer usage? I find it intrusive and insulting.
eta -- and while I have total sympathy for CG and his family, I find it a bit unnerving that they go to these lengths for the murder of a LEO...but what about all of the other people who are murdered? It's got to be very expensive to take so many samples and analyze them, and it is so very unusual. It really does smack of favoritism, as if the murder of one person is deserving of so much more in terms of money and resources than that of someone (anyone) else.