I also try to stick to speed limits, but anyone can make a mistake and either drift over the limit a bit or miss a change of speed sign, get caught out and receive a fine. In fact, someone who doesn't purposely speed is more likely to be caught out because they aren't as hyper aware of speed cameras as someone who knows very well that they are speeding. So who does this law affect?

Exactly. Speed limits are more about collecting fines than keeping people safe. How fast someone drives on a road is highly governed by the design of the road. Narrow roads with shrubs or tress on the sides make people drive slower. Problem is we build all our roads as wide as interstates with curves to match a race track. Then they post a super slow speed limit sign on it. It is not an issue of people being irresponsible, it is bad design!