It's the IMEI, and Apple is certainly not alone in helping - any wireless provider can help, and indeed there are services available to law enforcement to determine if a cellular device is registered to another wireless provider, and contact them. Every cellular provider can also blacklist an IMEI, and it won't be able to be registered on any network, effectively bricking it.
* Every cellular provider can also blacklist an IMEI
This is very country specific and requires the cooperation of all service providers. Once the phone leaves the country and registers on different network the block becomes ineffective. Thieves here know the phone will be blocked so they send it on to Russia, for example.
