The fact that the ability to hold more of a charge is something merely unlocked via a costly software update means Tesla sold you a 75kWh capable battery all along but gimped it artificially.
From the strict sense of "getting X when you pay Y amount", that makes perfect sense. (Tesla is essentially giving you a price break on a Model S60 or 60D by selling you the same car they normally charge a higher price for, and letting you pay the difference when you want to unlock that extra charging capacity.)
BUT .... when I buy something as expensive as a new car? I guess I expect all the physical equipment I get in it to fully function. Tesla is treating all of this like a computer on wheels that you buy and do various software upgrades to.
From Tesla's standpoint, I can't imagine they're actually losing money on every S60 or 60D sold, with the hopes those owners will eventually buy the software upgrade that forces them to pay back the rest of what the car was actually worth. The fact they offered these tells me that they can, indeed, sell the car at a reasonable profit with the 75kWh battery in it, but at the S60 or 60D price. Then, the rest is pure profit when those customers opt for the upgrade.
In the auto industry, the usual situation is -- any time a manufacturer artificially holds back some capability of a vehicle, the aftermarket finds ways to offer relatively low-cost ways to remove those restrictions. (Custom tuning of factory ECUs and transmission control units is a HUGE business.)
I'm wondering when we'll start seeing performance shops offering their own, cheaper unlock/re-flashes for Teslas?