Up to recently, ISP felt that they had the same status as traditional telecommunication provider and were protected from prosecution for the traffic going through their network. It was then none of their business to police the information flowing through their network.
The situation became hazier when BT decided to deploy [cleanfeed](http://en.wikipedia.org/wiki/Cleanfeed_(content_blocking_system). Up to that point ISP had been transproxying web traffic in order to cache the web page requested and save on bandwidth cost but had never actively interfered with the data passing through their network.