While there is obviously a good reason for this, 9-1-1 calls from cell phones, especially deactivated ones, are a huge pain for emergency dispatchers. The 9-1-1 system was developed with land-line phones in mind. A call from a land-line is easily traceable, so call centers can identify addresses and respond to emergencies quickly and easily.
Calls from cell phones are much more complex. The call centers can only trace the call to the cell phone tower that routed your call to them. At best, they can pin down a location within 300 meters of the source of the call, not much help if you’re calling from an apartment complex. Your call also might go to the wrong call center, and you will have to be redirected. People have actually died because emergency workers weren’t able to determine where their call came from fast enough. (USA Today has an article about a woman who died because she called 9-1-1 on a cell phone).
Deactivated cell phones are even worse. While it’s useful that people who need help don’t need to keep up with their bills, the calls are completely untraceable. There’s no phone number attached to the phone, and the phone service provider can at best only provide the last known address associated with the phone. Call centers have reported that children often call 9-1-1 when playing with their parents’ deactivated phones, not realizing that 9-1-1 calls still go through.
To learn more about how 9-1-1 calls work (and don't work) on a cell phone, read this CNN article. You can also read some horror stories that emergency dispatchers have about deactivated cell phone prank calls here and here.