US was not under any obligation. People of India asked to get rid of the food aid from US as well.. It was a decision of the govt to even ask despite opposition.Were the Us under any obligation to supply us the wheat. Remember as per the terms the payment was made in INR with the provision that the money would not be sent back to the US. Who would agree with such terms.
For a moment just turn the table around to Indo Nepal relationship. you will find the same grouse that the Nepalis have with India, the way we have with the US, despite India pumping so much money and aid to Nepal. We twisted their arms in 80s under Rajiv Gandhi for very flimsy reasons. So does it make India morally superior-- no we were just following our National Interest.
"What particularly irked the Americans was the Indian applause for a much smaller shipment of the Soviet wheat that had arrived much earlier"
Russians did sent the wheat shipment at the earliest even though it was small which they can send us. That's what one can call aid or friendship or courtesy or humanitarian or whatever, but certainly not 'screwing'. US sent the excess food supplies after a long long time.. That's the difference people would not fail to notice.
One of the successive PM's did say that instead of asking for aid, he asked the nation to live by cutting one meal a day so that everyone can have food. And the nation gladly followed.
If only feeding the entire nation was the priority, then India would have been chamcha of US a long time back. If only saving soldiers or saving the land is the reason, we would have surrendered before China in 1962 war. But there are somethings we don't do even if we have to face hardships. That's the principle on which India lives.