Why shouldn't it be acceptable?

You bought a 60 kWh car. If it performs as advertised, you not entitled for more, no matter what hardware is inside. And if you find a way to exploit the full capacity of the battery, you can, it's yours, but don't count on Tesla to support it.

I have no problem with market segmentation. It allows more people to have access to a product while protecting the margins of the manufacturer. Because, without margins, there is no product.

And in fact, I am happy when I am traveling and the student next to me paid half of what I paid, I could afford the higher price ticket in exchange for more convenience, so no problem for me, but if the student had to pay the same price as I did, he wouldn't have been able to travel at all. I was like him years ago, and our children will probably be too.

The problem in this case is that you're dragging along 15kWh worth of battery weight, which is a waste of energy.

Either way, the capacity lowers as time goes on. Your 60 kWh battery might become 50 in a few years. The extra 15 is supposedly a buffer against that.

It’s the same thing with spinning hard drives and SSDs. The reported capacity is almost always lower than the actual capacity. That allows dead blocks to be “replaced” without the user even knowing (“remapped sectors”). Could you imagine the PR disaster if your SSD lost visible capacity as time went on?

You do have a point about the weight. The cost of shipping an SSD with 0% overprovisioning and one with 25% is negligible; the ICs weigh just a few grams. However, a car that has to haul an extra 15 kWh of unusable capacity does use extra energy. I’d be curious how much is needed to haul those extra cells.

Sure, but using 60kwh of a 75kwh battery lasts longer than a 60kwh battery would. Not to mention that Tesla's R&D to make a 60 kwh battery, an extra asmembly line (or one flexible enough to produce both), extra inventory, testing, maintenance manuals, etc all has costs as well.

Cars are often shipped with disabled seat heaters, CPUs are often shipped with disabled cores. As a customer I don't see why should I mind.

I understand in case of Tesla you don't like the idea of hauling those extra unused cells around. But you are buying a car model with a specific mass, acceleration and range. It makes no difference whether how exactly the producer fit into those parameters.

Isn't that just contract law?

The problem is that the law is too abstract and not sufficiently exhaustive to address all the disingenuous schemes used by businesses.