Elon Musk isn’t known for modesty, and this week the Tesla Motors CEO laid out his ambition to grow 50% a year and deliver a stock valuation in 10 years of $700 billion—the current valuation of Apple, the world’s most valuable company. So why does Tesla, which is already valued at about $27 billion, still need so much taxpayer welfare?

Mr. Musk this week played down a $108 million loss in Tesla’s fiscal fourth quarter, which he attributed to one-time manufacturing inefficiencies, a strong dollar and lower than expected deliveries....