Because it's not free, it's $6000. It's fine to mention the two things together but rolling them into one figure is nonsense - expected operational savings don't have any impact on purchase price. In the context of the Tesla, the 'gas savings' are 1) an estimate based on a specific scenario and therefore not guaranteed, and 2) not materialised at the point of purchase; this is the difference between 'price' and 'cost'.

Try walking into a Tesla dealership, telling them you expect the car to save you $10,000 over its lifetime, and that you should therefore pay $10,000 less than the ticket price, and see what they say.