It's a much bigger leap to go from "punish people for speeding" to "physically prevent their car from going past a certain speed" than it is to go from "prevent this car from going past a certain speed" to "prevent all cars from going past a certain speed."
your slippery slope argument is not very convincing. What do you suggest be done (that isn't already) about people who regularly speed, putting themselves and those around them in danger?