Did it come as a surprise to you? Low profile tires are notorious for fast wear. When I test drove a Model S, the rep explained the differences between the 21" and the standard 19" tires, including the shorter lifespan.
The Model S is particularly heavy even for a large sedan, which combined with a lot of torque will result in extremely rapid tire wear even for a performance car.
The profile of the tires has almost nothing to do with wear. The treadwear (softness/stickiness) of the tire's rubber compound combined with alignment and driving style is what governs tire wear.
If anything, lower profile tires will actually wear less when driven aggressively as the sidewalls are less prone to folding (causing outer edge feathering) when cornering.
The biggest thing the Model S has against it for tire wear (both the 21' and the 19') is the 2° negative camber on the rear wheels when in standard suspension. While that is very useful for improved handling and preventing possible oversteer (especially likely given the huge amount of instant torque), it is hell on the tires.
P.S. While I'm an owner, I'm not that much of a car nut, and I am simply regurgitating the information shared by other owners who are car nuts. :)
