Would that not be massively unconstitutional? Surely law enforcement isn't allowed to use illegally-obtained records?
It's inadmissable in court, but that data can be legally used as tips, leads and to inform an investigation for more legal data collection avenues: it's called Parallel Construction: https://en.wikipedia.org/wiki/Parallel_construction
An example would be a police tip phone-number: it's all hearsay (unless it's a confession) and largely cannot be admitted into court directly, but it definitely leads to collecting solid evidence later on by investigators who can get warrants.
At this point were in national security letter territory.
The issue is that 97% of all federal court cases are settled before trial and never reach that stage because people often admit guilt and take a plea deal and don't know their own rights.
Also relevant: https://youtu.be/d-7o9xYp7eE