How could that be linked to you? Suppose my DNA is on the person I brushed on the street today. How would a government agent be able to prove that whatever DNA they extract from his coat sleeve is mine? In order to do so, they would have to have a sample of my own DNA. It is unlikely a judge would sign a warrant for my DNA, given that I am not even suspected of any crime.

You argue that a judge could issue a warrant on an innocent person. However, warrants are only issued if there is enough plausibility that a person could be guilty of a crime. At the point the warrant is issued, the government can now force you to give your DNA. But that could happen right now.

Ultimately, in this case, the FBI barely exercised any of their authority. They didn't do any backroom dealings or use 'secret' laws. They just signed up to a genealogy website where people freely share their DNA information with the DNA sample voluntarily given to them by a rapist and murderer. The rapist has no right to any kind of privacy after giving his DNA away. Moreover, no one has any right to prevent a person who shares their genetic information from not disclosing it.