It's significant that, rather than disclose their cell tracking methods, the police and prosecutors prefer to discard the evidence. This suggests that obtaining evidence is not the primary purpose of the tracking technology.
If the federal government doesn't want certain sources and methods exposed in criminal court cases against non-spy citizens, then the federal government mustn't allow the use of those sources and methods against non-spy citizens in criminal court cases.
The most plausible explanation here seems to be the simplest: If the methods were exposed, the prosecutors it would have probably been ruled a warrantless search.