I think mainly speculation, but see link below. I assume by "mandatory" they mean that your warranty (including the extended battery warranty) will be void without it. I doubt my local garage will have a clue what an EV drainage check on a Taycan is, so for now I think we're stuck with Porsche...
I think the high pricing, although obscene, is something you "buy" into. As above, no-one is forcing us to buy the car, and, by its very nature, if enough people are buying the car, then the pricing is correct for the brand. However, what really does annoy the heck out of me is being fleeced...