SSN and passport

I recently applied for a U.S. passport and, as an experiment, entered a SSN of 000-00-0000. The nice lady at the counter informed me that it would not be processed unless I provided a valid SSN (which gets forwarded to the IRS). It was my understanding that this disclosure was voluntary and that another form was available for notifying the IRS that a passport had been applied for. My travel plans didn't allow me the luxury of delaying the application process, so I provided the damn number. Does anyone know the real rules for this situation? Must passports be cleared with the tax goons or do they just want to keep tabs on people travelling abroad?