Re: IT's not all bad (sic).
Excusing it when it catches murderers (or terrorists, or pedophiles, or nazis...) means you excuse it for all crimes. Why not use your cell phone location data to see you were at point X at 3:00 and point Y at 5:00, and those points are 150 miles apart but the speed limit is only 65 mph on the interstate between those points. Here's your ticket in the mail!
If they had some reason to suspect them, filed a subpeona to legally get their cell phone data, and it provided corroborating evidence, that's fine. But if they arrested those guys because they "looked suspicious", then gained access to their location data without a court order I'd prefer that evidence was thrown out of court. Otherwise you are basically saying that the government can track anyone everywhere they go, and the only way they can avoid it is to choose not to use the single most important piece of new technology to come around in the last few decades.
If protecting my constitutional rights means some murderers get away, that's fine. A lot of criminals have got away scot free over the years because of the Constitution. If you want to catch them all you'll need to live in a place with a lot less freedom than we enjoy today. If you want to live there, don't fuck up the laws for me here. Find a country that already has those laws.