You are basically proposing a finder's keeper mentality.
In your case of a tow hitch, if the owner didn't pay for it and it was a mistake, it's still possible for the owner to retrieve it. This has been litigated multiple times. You buy a house, there's some hidden treasure that belonged to the previous owner. You are the new owner so do you own it? Not if the owner didn't intend to include it. New owner loses in court all the time and they have to give it back.
I disagree with your analogy. It is not a pricing mistake such as you were given a piece of physical equipment that you did not pay for. Instead is a manufacturer's decision to equip all cars identically and to provision options with software. A better analogy is an aftermarket tuning chip. A custom tuning chip that unlocks 25 HP from your existing engine hardware is not illegal either. On the contrary it is a thriving aftermarket. Why? Because it is your engine not the manufacturer's. If the performance chip also tunes the engine and the transmission shift points, nor is that illegal. Car manufacturers have tried and failed, to harass tuners. Now suppose it also tunes the active suspension mimicking an optional feature offered by the car manufacturer, or doing it one better. Still perfectly legal. As long as it does not disable emissions controls the government is completely uninterested.
Now if the same aftermarket unveils a "interior comfort chip" that enables say, one touch power windows or cruise control that would be just as legal as an engine chip.