There is an FAQ of this nature...
"My baby was born in America and I have his/her birth certificate. He/She also has an Indian passport and I have taken an appointment for him/her. Is this okay?".
on the website of US consulate in chennai.
See...http://chennai.usconsulate.gov/bepfaq.html
So, this makes us think that US consulate in chennai is aware of people going back to India on Indian passport and they are not imposing the rule for EXIT from USA. However, I think they are imposing the rule for ENTRY in to the USA.
When you leave the US to another country with a valid passport from that country - there is little the US Immigration can practically do - can they prevent you from leaving the country? To what end?
On the other hand when you try to enter the country they have to decide if they should let you in. So, you need a visa or proof of citizenship. Of late, proof of citizenship means passport.
This seems logically sound. Not that immigration laws are logical all the time...