Dual nationals may be required to present both passports, if questioned by the airlines or other officials. I've seen this occur while standing in line in Hong Kong. It was treated as no big deal as dual nationality is prevalent there, they just needed to see the proper passport(s) with the right visas.
All US citizens must lawfully enter and exit the USA by using a US Passport, if they are dual nationals. That is the US law, and it doesn't matter if there are no "exit stations" like in Taiwan.