That was back in the days when mobile internet meant WAP with services provided by your own network, i.e. the blocks were for adult content that O2 (and other n

That was back in the days when mobile internet meant WAP with services provided by your own network, i.e. the blocks were for adult content that O2 (and other networks) were themselves providing/selling. So yes, it was caused by the same law that prevents children from buying adult magazines or subscribing to porn websites - though likely the networks could have got away with less had they wanted to (such as the "If you are under 18, leave now" messages most porn sites now offer).