It's in the article. It was a criminal investigation. This is no different than subpoenaing phone records or financial records. They had a very narrow target and a specific objective well within their purview and serving the public interest. The subpoena was withdrawn because they caught the guy.
No: they wanted to identify one sex offender, probably because they knew he read this article somehow.
They did not care who read the article itself which was benign.
They wanted to create a precedent to make their job easier, not realizing maybe people care less abt solving crimes than making it forever possible to track who reads what.