PPT Book Fees Subsidize PPT Card/General Consular Costs
In today's Federal Register, State acknowledges that it subsidizes certain consular programs - including the artificially cheap passport card (not valid for air travel) with passport book fees. It also notes in another recent FR notice making its increases to non-immigrant visa fees final that these fees pay for general (not visa-related) consular positions and costs.
http://edocket.access.gpo.gov/2010/pdf/2010-15622.pdf