That means that the thief turned it off, and/or wiped/reset the device.
As soon as someone wipes and resets the iOS device, it's impossible to trace it anymore.
Sorry to hear that. I had two iPhones stolen in three years. One of which was last thursday...
There's a huge aftermarket for stolen Apple goods. It's easy to reset and impossible to trace if turned off and wiped.
While Apple and the telecom businesses may query the state of devices on a network at any given time (it would be hard to hack the IMEI, or unique device identifier), but won't do anything to help. It's too burdensome to help people like us who legitimately purchased an electronic device (sigh).
So for example, your iPad gets stolen, then wiped and reset with iOS on it, and connects on a wifi of cell connection. Theoretically, the device sends info as to what it is, probably including its serial number/IMEI. If Apple and the telecom corporations were any nicer, they would brick all devices which have been blacklisted through a centralized database. Not to mention the possibility of prosecuting the thieves, since they are traceable after all...