Quite simply, it is not the providers responsibility. AT&T has no (or very little) input into iOS. Apple is solely responsible.
Apple provides MobileMe which includes the "Find my iPhone" functionality.
No carrier in the US does IMEI blocking and there are no laws requiring it.
I can only imagine the frustration and anger you are experiencing after having your device stolen, but ultimately it is up to the user to ensure there is a passcode lock on the device and the "Find my iPhone" app is set up on the device.