Judgements in a common law jurisdiction are based on reasoning about how the law applies to the facts. It is a fiction to believe that there is a clear delineation between law and fact - the facts inform the law and vice-versa and the means by which they do so is the reasoning of the judge(s); that is how precedent works.
Furthermore, theoretically (see above) an appellate court does not make findings regarding fact; an appeal can only be based on a mistake in law. That is, an appeal is an argument that the lower court made a mistake in reasoning about how the law applies to the facts.
So, on that basis, the report is being used as a legal source.
I will venture to suggest that the way the report was used was to inform the reasoning process that court used. The justice is effectively saying: "These learned people who have looked into the general expectation in the USA regarding privacy of communication and have said this; I agree with what they say so I will use that as the basis for my reasoning." This serves two purposes: it shows us what the justice was thinking when he built the links in his judgement and it shows us why he was thinking this way.
When you think about it this way it becomes easier to understand how and why multi-judge appeal courts can have split decisions. The facts are the same to each judge, the law in the form of statute and precedent is the same to each judge but each judge's reasoning process is different. A different process leads to different results.
A judge can consider anything a legal source, in the sense that it informs their decision making process.
Judges can and do look to many things to inform their reasoning process. They will often look to:
- speeches made in legislature
- cases in other jurisdictions
- commissions of enquiry
- academic papers
A judgement must be based on the law and the facts and decided on a basis that the parties argued (deciding on a basis that was not raised can be a failure to afford natural justice). Furthermore it needs to set out the reasoning process in sufficient detail, that:
- the parties can see why they won or (especially) why they lost
- an appeals court can consider if the reasoning was sound. It is worth noting that while a case decided in the Supreme Court is not subject to appeal; the precedent created may be overturned by the Supreme Court latter on and so it is vital that that group of Justices knows what the hell the previous group was thinking. (Is there a collective noun for judges?)