Why do they make consumer automobiles for use in America that can attain speeds well over the legal speed limit (e.g. 140mph)?

Many consumers wouldn't purchase cars with capped speeds. Americans like to speed (or at least the ability to speed).

I would assume also to justify price and simply for marketing. I also think the it keeps interest going for consumers.

Lots of cars are engineered for quick acceleration rather than highest top speed. That's why companies advertise the 0-60 time, not the 0-120. But the two go together -- an engine powerful enough to accelerate that fast will also have a high top speed.

Some people with fast cars drive their cars on a track. Sort of like private racing, except the drivers aren't racing each other. It's safer and probably more fun than driving fast on a straight highway.

Besides, if they capped speeds via the ECU, two days after the car’s release there’d be a reflash to get around it.