I think they sometimes feel compelled (or have an agenda) to fill-in the blanks when there really isn't enough information to give a complete answer.
I don't think the SCOTUS justices have that much of an agenda. They've spent decades devoting their lives to legal analysis and constitutional interpretation. They have a belief in how the constitution should be read, and they stick to it even if the result is more liberal or conservative than you would expect. That is what I love about them. I've read cases where Scalia and Ginsburg write the Dissent together. Or where Scalia is all about preserving privacy interests and limiting the scope of warrantless searches and Stevens is saying that a warrantless search should be allowed in that particular situation. I've read cases where the originalists' attention to text and history creates an asinine result. Sometimes the living document opinions make the most sense. I think that might be why I like the fence straddlers the most.