Why does it matter? You not allowed to go fast with all the bl@ddy speed cameras....
Anyhow. Since many of us disregard the camera's

...
I think the reason is mainly because they dont want to go out of business. Think about it. If Volkswagen produced a Golf GTi that was getting the most out of its engine standard then:
1. What happens to the countless people that go remap that engine further and damage it? (VW dealers will turn into a parts store and not a dealer)
2. What will we have to look forward to for the next generation of that vehicle. People wont buy it becasue it looks a bit different. They buy it because its got a improved and faster engine.
Look at BMW. We bought one of the first 3 series to come out. It was useless. Engine was pathetic. Car was pathetic. 3 years later they are still stocking the same car same shape but when we got the exact same vehicle on loan it was like a differant car.
A car manufacturer wants to keep its buyers on its feet. It launches the GTI.. WOW then people start getting bored and whoops... They R32 is launched

3. Also because with vehicles there are rivals. Example. Volswagen produces a GTi with a V8 engine

so what will ford do? They will launch a ST with a V8 that will be mapped too and it will tear the GTI apart sort of thing... Then what? What can Volswagen produce to entertain us?
4. Buyer market. Think of what market the GTi is aimed at... (not slating Phil) but look at the Passat sport. Its aimed at the man that has a family to big to accomodate the GTi. They gave it that power through whatever engine and map so that it would appeal to him.
I think in general the main reason for Manufacturers not overpowering there cars are also laws. Certain countries have certain guidelines for certain engines.... I.E: A 2 litre FSI engine is only allowed to produce so many of this carbon and this fume blah blah. So if they manufacture the car with a 2 litre engine they have to comply with the laws with regards to 2 litre engines else they end up upgrading the engine top something better (V8) and then as said perviously the target market, rivals ect all goes out the window.
I may be wrong so I stand to be corrected.
Mike