The dilution is peanuts compared to the premium they paid in the first place for Elon Musk's reality distortion field.
edit: to further explain what I have apparently not (judging by the speed with which I got the first downvote), what I am saying is that Tesla trades at a massive premium to any normal way of valuing a public company - far more than the dilution of another offering. Clearly, the people investing in Tesla are not worried about dilution, and are betting on the Big Plan and the ability of Elon Musk to create something absolutely freaking massive. Either that, or they're just not doing the math. But I'd bet on the former.