There are Federal rules as to what the minimum average MPG of a car manufacturer's fleet must achieve.

By selling one FEH that exceeds the minimum MPG ( by a lot ) Ford can sell 4 or 5 gas hogs that miss the mark ( by a little ) and still have a good average.
So there IS incentive to sell hybrids, even at zero profit margin.

Have you seen the Honda ad that says "World's most fuel efficient car company."
Due in large part to the 66 MPG Insight. But only a few 1000 Insights have been sold.

Certain "Heavy Duty" and commercial vehicles are exempt from the average.