That is not actually true, and I am saying this as a dual citizen who is culturally American. Under 8 USC section 1185(b) it is unlawful for any Untied States Citizen to depart from or enter or attempt to depart from or enter the United States without a United States Passport. So you need an US passport to leave or enter the US even though you do not show immigration agents your passport when leaving the US. When you return you will need your US passport to be admitted.

While it is true that there is no "passport control" on exit in the US (while there is in Europe), it could be easily implemented and scaled into action by a simple law passing Congress.