The laws and legal opinions are public domain. The issue is distribution. Pre-Internet, courts were not in the business of publishing books of their opinions. Making the opinions available at the court house was deemed to be sufficient public access. That treatment was consistent with pretty much every other kind of public record (you had to go into the county clerk's office to view deeds, etc). Post-Internet, many courts publish their opinions online. E.g. the Virginia Supreme Court and Court of Appeals has opinions going back to 1995: http://www.courts.state.va.us/opinions/home.html.
What private parties provide is a value-add. That's what you purchase access to. They spend the money and go to the effort of digitizing, indexing, and annotating, case law and statutes going back a couple of hundred years. No single public entity can do that, because the 51 individual federal and state court systems are part of 51 distinct sovereign entities, and as a practical matter individual courts even within a single sovereign operate largely autonomously of each other.
What private parties provide is a value-add. That's what you purchase access to. They spend the money and go to the effort of digitizing, indexing, and annotating, case law and statutes going back a couple of hundred years. No single public entity can do that, because the 51 individual federal and state court systems are part of 51 distinct sovereign entities, and as a practical matter individual courts even within a single sovereign operate largely autonomously of each other.