And that's it exactly. The stingray is basically a wireless phone tap without the pesky need to go through the phone company to do it and therefore no need to goto a judge for a warrant or even an NSL letter.
Right now they can tap anyone they want whenever they want with no oversight whatsoever. They don't want this brought before a court as this would reveal their (probably illegal) parallel construction schemes. So it shouldn't be a surprise they'd rather throughout a case than have it brought up for review.