If mobile networks can do this then where is the need for the interception of traffic, filtering and - in Vodafone's and 3's case - the sharing of URLs visited with companies located abroad? It would appear that the filtering that this is supposed to support is not necessary.
Surely all that need be done is limit the sale of SIMs and other internet enabled devices to adults and let them decide what controls if any are required? If no such controls are available on the device level then maybe - just maybe - parents shouldn't be letting their precious little brats run riot with no supervision online with said devices in the first place?