There's still no nationwide database in the US of all stolen IMEI numbers
Actually there is. The two major GSM carriers, T-Mobile and AT&T, share a database. Sprint and Verizon will be joining that database by the end of the year; though not that stealing a CDMA phone does you much good on a GSM network and vice versa at the moment. In any case the problem is that the IMEI database is not enough;
- IMEIs are not unique. We've hit the equivalent of IPv4 space exhaustion. So they're simply reusing IMEIs now.
- IMEIs can be changed on a number of phones, so it's not a reliable way to keep a phone blocked.
- These IMEI databases are not shared on a global level, and there's really no way to force everyone to work together. China Telecom for example has little incentive to block iPhones stolen in other countries
The solution then is that rather than merely unreliably blocking a phone, the phone needs to be disabled entirely so that a stolen phone cannot be of any value. It essentially needs to be (reversibly) destroyed if stolen, to eliminate all financial incentive for stealing a phone. This is why the Attorneys General and other law enforcement officials want kill switches, so that shipping a purloined phone overseas is no longer a viable business, ultimately leading to criminals to stop stealing the damn things.