The claim is that it is cheaper to make them the same than to have different manufacturing to the different specs, so they would be (and were) putting the hardware in there anyway but turned off to differentiate between trim levels. Dunno if it's true but seems lots of car companies have done it over the years so I assume so. To me the issue is the wastefulness of trying to create fake differences between trim lines in that case, if you are putting it in anyway then just include it in the lower trim.

I know why they don't, as people will pay to get some features and if seat heaters are on the "important" list then they definitely don't want to give it for free when it can be a hook to buying a whole bunch of crud you don't really want.