Because they had the buyer underwrite the cost with the purchase. If it had intrinsic value at purchase time, that cost should have value later, too. I am sure their margins on cost of development increase over time, so if they charge a current buyer
That would be assuming that there are no new development costs for Tesla and that they've already paid for the development cost that they've encouraged. Neither one is a likely scenario. Development cost will continue, and the proprietary software that they develop has value. If it has enough value for a buyer to pay for it, then it has enough value for Tesla to reimburse it.