But the GTR is speed limited to 180km/h aswell.
Both cars can reach way above this with the limiter removed. Top speed of a vehicle is determined by the top rpm the engine can reach or is rev limited at multiplied by the gearing ratio in the top gear of the car (5 speed/6 speed), then multiplied by the final gear ratio in the diff (3.73:1, 4.11:1, etc). This gives a hypothetical top speed of the vehicle, however the vehicle then needs the power behind it to over come the force from the air pushing against it trying to slow it down. Hence why the Veyron and other cars need so much power behind them to reach over 400km/h.
Looking at this, you can see that most vehicles can reach quite a high speed, especially ones with tall gear ratios and a lot of power, therefore the only logical thing to do is limit them to a certain speed electronically to stop people pushing the car to its top speed when the rest of the car (tyres/brakes/aerodynamics/supsension/cooling) aren't designed to withstand that high speed and strain on the vehicle.