Key: Sells parts for. Tesla is selling parts here. The question is why they are charging identically for them for cars that are in warranty. It's awesome that Tesla is supporting old cars, and nobody expects that to be free. But I do expect my 2018 car to still be working in 2021 without having to pay Tesla more.
A key difference here is that parts and service are a huge profit center for dealerships.
The parts aren't made by acura, they're made by another company (Delco / Bosch / Denso / etc) according to the specifications from the OEM; they make the same part and sell it directly as well as sell the part in an acura box and sell it to/through dealerships.
Similarly, the dealerships make big money off of servicing their older fleet of cars. I can't remember exactly where it was but I remember an interview with Musk where he referred to this legacy income stream of supporting old cars as a huge moat protecting legacy auto sales from a newcomer -- who would have to generate a decade of new cars before ever being able to make a recurring profit off of selling parts / service from the older cars.
For tesla, as of now, the service centers are a cost center (probably burning huge amounts of money) while for all other incumbent auto makers the dealerships are an indirect asset; they can use them to hide mistakes (hey if you want the C8 Corvette you need to buy a bunch of malibus; we'll make it up to you in the next quarter).
Finally, I suspect tesla simply hasn't gone through the pain of a vicious recall to the point where they're gunshy about being conservative in their supply chain. Every part you make is a potential liability; they understand this with batteries because they promise to support them for 8 years, but they're probably starting to realize they've got these safety liabilities that extend even past that.
But, hey, they're a smart bunch and have enough money in the kitty to whip up a solution to this. It's just going to take them some time. But now at least they've got incentive to actually fix it. Probably they'll also fix the wonky batteries they've made -- hopefully with a LiFePO4 pack...