Others have focused on the psychological effect of the practice, primarily with relation to sales. But I'd like to focus on what one should think of with a speedometer that is free of these sort of plots.
Different countries different rules
In places like Germany or the Isle of Man, there are highways on which there are no speed limits.
Different countries have different speed laws. Having to tailor each car to its target market would increase cost and distribution complexities.
Rules can change, the system doesn't
To continue the point above, imagine the state of California decides to remove speed limits on some roads - this could cause problems with some cars whose speedometer was designed with some local speed restriction in mind.
Rules are sometimes broken
Imagine your friend got stabbed, he's bleeding heavily, and you must rush him to a hospital. There are no ambulances in your village and the hospital is 10 miles down the highway. It's 3AM and there are virtually no cars on the road.
When the life of someone is at stake, even the most law-obeying citizens might break the speed limit.
Speedometers also communicate limits
While main role is to communicate speed, the speedometer is also representative to the limits of the system, basically telling users "You are this far from what this system can handle".
This is an important point - whatever the normal range of speed is, it is still important for users to know what their car is capable of. In most cases this is important for marketing, but there's also a matter of safety here.
If the limit set by speedometer is a false one, you risk people pushing the car over that limit ("it feels OK, surely the car can handle this"), with no indication when they approach the real limit.
Consider these two speedometers:
Driving at 160 MPH should be perceived slightly differently on each car.
Now all of the concepts above are surely to be muddled by inconsistencies and marketing plot. But if you exclude the latter, or any other psychological design decisions, it is important to see the speedometer as a system-first type of device.