They usually have a guy with a helmet cam roaming about, mostly for games when they think the atmosphere could turn nasty.
I wonder if by entering the ground you are agreeing to having images of yourself captured in this way?
There are cameras everywhere in the ground anyway, pretty sure that box suspended above the away fans is a set of people constantly monitoring the crowd.
The policemen with cameras are just there so that fans know for certain they are being filmed - it's a crowd calming measure.