More like Audi/BMW putting a 250 km/h speed limiter on the car you just bought. Sure, you can go ahead and remove the limiter yourself, and why the hell not change the fuel mappings on the ECU while your at it? Audi/BMW will not support the modifications nor honor the warranty on your car, but there's nothing 'physically' stopping you from making the modifications. They are by no means obligated nor legally required to tell you how to circumvent their limitations and reverse engineer their software.
When an engine suddenly catches on fire doing 270 km/h+, or you suddenly loose control on the car, the last thing they want is for you to point the finger at them and say: "Well you technically allowed us to do this". They are just doing everything possible to cover their asses.
Look at Windows Mobile for a minute. Stock installs are actually quite decent. But when Joe Sixpack starts installing "Bubble Popper 2.0", and "FREE XXX PIX" on his phone, and the phone shits a brick, guess who takes the blame? Yeah, Microsoft and their "damn unreliable OS".