impossible to block

Mobile phone companies pre-fetch pages as standard before allowing some pages to display certain words. (And most unlock it almost immediately to do normal stuff as the filters block nearly everything)

On broadband the system is very different, it would create chaos to implement and drag decent networks to a crawl.

And finally its impossible to block anything on the net, a tiny encryptor and plugin for a browser would render the block ineffective (works well in china)