As far as I know, a warrant is required to tap a landline phone. I don't see how the principle behind that (reasonable) interpretation of the 4th amendment would not also apply to any kind of data (or metadata) from Internet traffic. Just because some 3rd party collected what is, by legal precedent, clearly considered personal effects and papers does not mean that 3rd party can grant absolution to the government in its need for a warrant to obtain that same data.