If you bought the car because this fulfilled your needs, are you now being robbed because there theoretically is more horsepower available?

Your premise is flawed. The horsepower didn't become available now; it was always available from the beginning -- the physical machine didn't magically change. That means even the most charitable interpretation is that VW deliberately made the thing artificially worse when they sold it to you.

Are car motors not always limited to specific power outputs to reach emission, efficiency, or safety targets?

Sure, but the bottom line is that either a tune falls within those targets or it doesn't, and a tune provided by the manufacturer always will (because they have to conform to emissions laws, honor warranties, etc.). Since the higher-performance tune is safe, using the lower tune is just leaving performance on the table for no reason.

It is not like a tune done by the owner or third-party that could exceed those limits at the owner's risk.