Here is a good link that explains why LE cant really trace anything with a prepaid burner phone.
Simply it is because you don't put any personal information on the phone. The service just gets activated and you buy the phone with cash if you want.
LE cant get a search warrant because there is nobody to ask a judge to get search warrant for to get the records. And even if they knew the phones ID then nothing would hold up in court because LE cant prove who had the phone in their possession. You can give the phone to someone else if you want.
If LE could go to that provider of the phones and knew the store location then they may be able to find all phones usage from that store. But that would take lots of money and research and no judge will allow search warrants for something like that because its all speculation that they would be using the phone for some illegal reason.
These are the types of phones that get smuggled into prisons.
http://www.makeuseof.com/tag/sick-of-the-nsa-tracking-you-burn-them-with-a-burner-phone/