It's all about national pride
Nokia, even in the strongest of its years, never had the presence in the US that it did in the rest of the world.
Neither the Nokia or Microsoft brands will warm the hearts of the Chinese, but the Microsoft/Windows brands will appeal to Americans. It's a very simple decision for them. Use the Nokia brand in China - where at least the distribution gives them the edge. Strongly use the Nokia brand in India (where the word "Nokia" is a colloquialism for mobile phone: "I'll call you on your Nokia", even if the phone is a Samsung), but push the Nokia name right to the back in the US and promote the Microsoft name. Even on the S40 phones.
This regionalism is completely normal. Look at how - when there were plenty of players - Ericsson dominated Sweden, Siemens Germany and Sendo the UK. Hmm, perhaps not that one.