I theorize that Nokia was either going to switch to Android or was on the verge of going bankrupt. (I suspect the latter: part of the deal included €1.5 billion in financing available to Nokia immediately, and it’s decidedly odd that Microsoft bought Asha but not the Nokia brand or maps; it’s as if they were keeping the price as low as possible).
And so, I would argue that this deal is not unlike the Motorola one, where I believe Google had its hand forced by Motorola’s threat to sue other Android OEM’s for all they were worth. Microsoft didn’t have a choice.