“I would have bought my Tesla even without the tax rebate.” So went any number of irate emails from readers over the years, nonsensically for any point I was trying to make. Elon Musk isn’t in the business of leaving money on the table. He priced his cars to make sure every penny of subsidy ended up in Tesla’s pockets and not its customers’.

Now Wall Street finds Tesla sales are not adding up as hoped this year. Morgan Stanley is forecasting 344,000, below the low end of Tesla’s last predicted range. An obvious culprit is...