In August, Ancestry revised the terms and conditions on its site to make it
    clear that its services were off-limits "for law enforcement purposes"
    without a legal order or warrant

Good. This is the minimum I expect from Ancestry (I have >30k names in it). If cops want to leverage the data compiled by volunteers, get a warrant and get the oversight that comes with that.

    a legal order or warrant ... can be hard to get,
    because of privacy concerns. 

"A legal order" implies a subpoena; they require a signature from a magistrate. This is typically the opposite of hard to get.

A warrant can be hard to get if police do not provide sufficient cause or are asking for and excessively broad scope.

The difficulty level of getting a warrant goes way, way down when police submit the information the judge needs to properly evaluate it.

But police really really prefer to search without agreeing to boundaries. Including cold case officers, it seems.

    “Everyone who does this work has depended on the records database
    that Ancestry controls,” said David Gurney, who runs Ramapo College’s
    Investigative Genetic Genealogy Center in New Jersey. 

    “Without it, casework is going to be a lot slower,
    and there will be some cases that can’t be resolved at all.”

No where in here is there even a hint of acquiring the consent of the people who's data this is. Nor does the word consent appear in the article.

People who don't even think about getting consent are poor choices to be in positions of power or accountability.