That's an evolving situation, and a plain reading of the 4th amendment makes this whole process look not compatible.

Then the courts are wrong, as they often are when it comes to constitutionality. Domestic dragnet surveillance is a very obvious violation of the 4th amendment.

Id be interested in why courts believe that it is not. Is it because it's at the level of signals rather than correlated to people?

Multiple revelations including Snowdens reveal that the data is not uncorrelated/anonymous, if it were I'd imagine it'd be of little use for intelligence purposes. Maybe I am too dismissive/cynical but I don't put much stock in what the courts decide or why on issues like this, they're all under somebody's thumb from what I can tell. They're the same folks that brought us such hits as "corporations are people and money is speech", call me crazy but I don't think they're making their decisions based on high-minded constitutional principles

In what sense are they “wrong”? You don’t agree with them. But theirs is the final word on the matter until a new court or new law comes around. The interpretation of the Bill of Rights has varied greatly over the last 250 years even though the words have stayed the same. The law isn’t based on our ideas of what it is, the law is based on what is enforced.

Legality has nothing to do with any notion of "should".