The constitutional challenge was dismissed because it was found to be constitutional under existing jurisprudence. See discussion in the lower court's memorandum opinion and order starting on the bottom of page 38[1].
Smith v. Maryland, abridged, said that we have no privacy interest in the metadata sent to a third party in order to complete a call. In the words of 4th amendment jurisprudence, we don't have a reasonable expectation of privacy in items we voluntarily disclose to third parties. As such, the collection of this information by the government is not a search for 4th amendment purposes, and therefore falls entirely outside 4th amendment protection.
THE ACLU's constitutional argument is a novel one that has not yet become binding precedent on lower courts, call the mosaic theory, though it is gaining ground in some recent Supreme Court decisions. See Orin Kerr's paper on this theory of a search.[2]
> In the words of 4th amendment jurisprudence, we don't have a reasonable expectation of privacy in items we voluntarily disclose to third parties.
I would hope that thinking on this evolves, to the point that we have a reasonable expectation that information we've voluntarily disclosed to a third party stays between us and that third party. Because that is exactly my personal expectation, notwithstanding my other expectation that it will be violated.
Mosaic theory doesn't go that far, but in the age of widespread statistical inference, it is an important development.
Generally speaking, the public is entitled to all evidence. Certain rights, such as the Fifth amendment, protect you from being compelled to testify against yourself, but you have no right to prevent, nor does anyone else have the right to refuse, to testify against or about you, except for a very small and declining set of common law privileges, and even those only apply in certain circumstances.
Indeed, and there's where we need mosaic the most. As it stands, the collection of this type of data (Smith) is not even a search, and therefore the court does not even consider whether or not it was reasonable. It just stops there. If it were found to be a search, it might still come in, but perhaps for a different reason. There's a great cartoon, which contains this awesome flowchart: http://lawcomic.net/guide/?p=2256
That is a matter of law, not thinking. We have specific laws in cases where you can expect that information you disclose to others should be kept secret. Attorney-client privilege, HIPAA, etc. That alone should tip you off that types of communication without such restrictions are not protected.
Smith v. Maryland, abridged, said that we have no privacy interest in the metadata sent to a third party in order to complete a call. In the words of 4th amendment jurisprudence, we don't have a reasonable expectation of privacy in items we voluntarily disclose to third parties. As such, the collection of this information by the government is not a search for 4th amendment purposes, and therefore falls entirely outside 4th amendment protection.
THE ACLU's constitutional argument is a novel one that has not yet become binding precedent on lower courts, call the mosaic theory, though it is gaining ground in some recent Supreme Court decisions. See Orin Kerr's paper on this theory of a search.[2]
[1] (pdf warning) https://www.aclu.org/files/assets/order_granting_governments...
[2](pdf warning) http://repository.law.umich.edu/cgi/viewcontent.cgi?article=...