Well, the NPT itself is a carte blanche to US, USSR, UK, France, and China. The NPT gives carte blanche to all nuclear powers prior to 1969 and India tested in 1974 and many /signed/ the NPT in 1992, like China and France. That said, like any legal document the NPT has loopholes, or at least ambiguous wording, and just like the wealthiest lawyer wins, the wealthiest country wins. The U.S. decided to re-interpret the NPT from "not collaborating with nations outside the NPT on nuclear matters" with "not collaborating with nations outside the NPT on /military/ nuclear matters" and gave a green-light for selling nuclear fuel and technology to the civilian sector in India (which consequently frees up India's domestic nuclear resources for military use if they can import nuclear fuel and tech for civilian use). And once the U.S. gave that interpretation, Russia, France, and soon Canada and Australia will also adopt that interpretation and begin exporting nuclear fuel and tech to India for civilian use. Australia and Canada are big since together they have 80% of the world's uranium deposits.
In the end, it's all big chess game. What was the point of the NPT? Choices like "world peace" and such are nice for elementary school kids, but the reality is that the NPT like everything else is done to win, and in this case to maintain status quo for the major powers so they remain major powers. Then why be flexible and allow India? Because rigid structures are more prone to break than flexible structures. India became the 3rd largest economy ahead of Japan this year on purchasing power and by 2050 both the economies of China and India will independently surpass the U.S., and combined surpass the U.S plus Europe. Moreover, the U.S. doesn't see any long-term conflict with India, and in fact sees India as an ally which has a democracy, a liberal society, and a focus on business and economy rather than military. While India has nuclear and missile programs, its military budget is tiny, at only 2.7% of GDP, compared with 2.6% for England, 3.9% for Russia, 4.7% for the U.S. and 10.4% for Saudi Arabia: http://en.wikipedia.org/wiki/List_of_countries_by_military_expenditures and http://en.wikipedia.org/wiki/File:Military_expenditure_by_GDP_2008.png
All that said, it makes sense to slowly induct India into the status quo than risk a change in the global order. Every exclusive club has to occasionally induct new members to keep from turning irrelevant. That said, while a country club may accept a rich black man with the changing of the times, it's not a free-for-all where it accepts a homeless man. So the nuclear status quo will