Think about it from a supply/configuration/logistics point of view. It might be cheaper to make 500k cars with 100% heated seats rather than 250k w/ heated and 250k w/o heated. Even if BOM cost is lower, the additional complexity due to the logistics and supply chain probably means it's more expensive to have both variants in production. That way people who want heated seats are subsidising the lower overall cost for those who don't. It also allows the next owner of the car to pay for that option at a later date if they want to. Almost every industry does this (there was a good case about oscilloscopes a while back, where the cheaper, lower bandwidth models just had a low-pass filter circuit installed).

It makes sense economically, but still feels paradoxical. A similar example would be building houses with 3 bedrooms, where 2 bedrooms remain locked and inaccessible unless you pay for them. So a significant proportion of the population ends up living in 1-bed houses even though there is no scarcity of resource.

You're putting the cart before the horse on the heated seat argument though. It's cheaper to have one factory line for the seats. Ideally you would pay the same price for the car without heated seats whether or not the hardware was physically there.

It's the same thing they ran into with the software locked batteries. The goal is to have minimal overhead to try to drive costs down on Model 3, and making the SR just a software limited version of SR+ actually ends up pushing the overall cost down, rather than having to set up an entirely different line that does the cloth seats and the aluminum roof.