I'm going to guess it's because when you buy a car, you don't buy it from GM. You buy it from the dealer. That $1,500 option can become whatever inflated price the dealer puts it.
While if it's a subscription from GM, it's $1,500 and that $1,500 directly goes to GM.
While if it's a subscription from GM, it's $1,500 and that $1,500 directly goes to GM.