There's also no way they can stop all drug deals, bank robberies and murders, but they still legislate against those things, and I believe that legislation affe

There's also no way they can stop all drug deals, bank robberies and murders, but they still legislate against those things, and I believe that legislation affects people's behaviour. People will probably always be able to get together in secret underground clubs where they play with one time pads for kicks. This is not what the government / MI5 are seeking to prevent.