Not sure what it's like in the US, but in Canada, typically, if you want summer tires on a factory delivered car, you'll most likely be upgrading the size of the wheel too. Usually the "summer performance" option leaves you NO option.
North American consumers have been led to believe all-season tires are just fine, thank you. Besides the car nuts who know better, or people who just max out their config, there isn't a huge demand for summer only tires - certainly not up here (for some obvious reasons).
Not sure if this is Tesla's reasoning or not. Either way, I agree with you. They should throw in another choice at the 19" size for summer.