$18 per month over 10 years is $2,160. MSRP for the 2020 2 Series--BMW's cheapest sedan--is $35,300.
So instead of charging $2K--5% more--for heated seats out right they have no gone through the expense of (1) building the software, billing, and support infrastructure to manage this (2) pissing off customers.
I'm assuming they're also streamlining production by building heated seats into every car instead of selective models, but again, why not just go with a permanent upgrade model? Everyone buys heated seats, BWM provides a special chip to activate them.