(...) makes no sense to me is why should you unregister your phone when you sell it or give it to someone? How does fighting illegal smuggling
I believe they took that measure to cover this scenario:
A smuggler imports one device legally, its IMEI is therefore whitelisted. The smuggler starts cloning the valid IMEI onto his smuggled handsets. All the phones are now whitelisted. Sure IMEI cloning isn't something anyone can pull off, but I'm guessing it's worth the effort given the economic incentive.
I still don't think it's worth putting us, the consumers, through all the hassle. Given how corrupt our institutions are, it won't take long before smugglers start bribing the people who maintain the whitelist, paying them 5% (and passing that cost onto the consumers) and saving 10%. We end up paying more and dealing with the hassle of additional bureaucracy and reduced privacy.