My 3rd BMW and after 7 years on BMW forums, this is the first I've ever heard of this.
1. If this really needs to be done, it should be done under warranty, however, I've never heard of it needing to be done on a newer vehicle.
2. How did they know this needed to be done?
Sounds like BS to me. Plus leading with the whole "Hey, it's your lucky day, we have a special going right now, with this coupon you get $100 off". That's just dealership snake oil.