Regulation in some countries requires ISPs to block certain sites to all or some of the subscribers. Just recently we saw the legislation efforts in the US (here) and a court order in the UK (here) to block sites that feature copyrighted content, but the most common case is the global effort to stop child abuse content, or protect minors (and other people) from accessing inappropriate content. Some countries even censor sites for political or other reasons (imagine that!).
There are many ways to block sites, but as the UK regulator, Ofcom, concluded recently that "All site blocking techniques can be circumvented" (here). Nevertheless, regulation and possibly customer demand (see "Parental Control Opportunity: 87% Say they Need it, Only 43% has it" - here) drive vendors to offer a more robust technology that includes real-time categorization and verification of content.
The two vendors announced that " .. Mobixell Seamless Access Mobile Internet Gateway, featuring Commtouch GlobalView™ URL Filtering offers advanced content filtering capabilities which enable telecom operators to improve security for mobile device users and ensure compliance with child protection Web content legislation.. enabling telecom operators to block access to Web sites deemed unfit for children and comply with even the strictest child protection regulation. Content categorization from Commtouch is available as a module of the Mobixell Seamless Access Mobile Internet Gateway which enables telecom operators to manage complex content delivery policies, offering a variety of opt-in user protection services, including parental control, real-time rating and content verification"
"The content categorization mechanism within the Mobixell platform is powered by Commtouch’s GlobalView URL Filtering solution and the cloud-based GlobalView Network. The technology continuously monitors and analyzes Web traffic to provide real-time, accurate categorization of URLs from anywhere in the world. The solution accurately categorizes dynamic user generated content and Web 2.0 sites".
See "Protecting Children from Mobile Porn Gets Push from National Governments" - here.