From Wikipedia: "For example, if a mobile phone is stolen, the owner can call his or her network provider and instruct them to "blacklist" the phone using its IMEI number."
Is it because it's actually mutable/not properly authenticated? Or because global blacklist synchronization isn't good enough and not all operators respect them?
They generally don't work across national boundaries. A large fraction of the market for phones stolen from California is overseas.
It isn't farfetched to assume that network providers don't really care about stolen phones in the third world. Until they do, IMEI blacklists won't mean much.
Network providers probably don't care about stolen phones in the first world. If they could find a way to ignore domestic IMEI blacklists without too much public outcry, they probably would.
In the US, the top four carriers make almost $8 billion per year selling theft insurance to their customers [1]. I don't have a cite handy, but I recall reading that they also make a huge amount selling replacement phones to people whose phones were stolen.
Samsung planned to do a kill switch on its phones to fight theft a while ago, and the carriers blocked it. Many believe they blocked it to protect those insurance and replacement phone profits.
[1] http://www.huffingtonpost.com/2013/11/20/iphone-kill-switch_...
Background info here: http://www.moremag.pk/2012/11/08/3-million-mobile-phones-in-...
I worked closely with carriers and they would resist any attempt for you to do anything in their system and insist it wasnt possible unless you had the cops or the legal system behind you.
[1] http://www.staffordshire.police.uk/info_advice/crime_prevent...