For the last few years CCTV prices have been dramatically falling, quality has been going up and what a CCTV camera can actually do is only limited by what we can think up. Coupled with huge amounts of section 106 money and companies wishing to buy good will it was obvious that the Government followed by local councils would pick up on money making ideas. I would love to say they are for our security but as the police almost do not exist anymore I think we can rule that reason out.
The reasons for using lamp posts and traffic lights amongst some locations you would not have considered is that there is power to run the cameras and internet links. Although cameras have come a long way the best results are still with the aid of some form of lighting. A lamp post is probably the best location to place a CCTV camera.
At the moment WI-FI is a fairly secure form of transmission although there are already reports of hackers breaking in to signals.
Given the area we live in and the people who serve us I assume their use will not be just for making money but also for an extension to the "Big Brother" conspiracy.
There are many links associated. This is quite a clear one,
http://www.thecloud.net/posts/archive/traffic-lights-and-cctv-cameras-to-be-used-for-wi-fi-routers/