It was not a rational decision.. it was emotional. Nokia executives hate passionately Android since it killed Symbian.
The only differentiation Lumia 800 offered when entering market was the ipod-nano-curvy case. It was simply are more marketed version of Samsung omnia 7 and HTC HD7 (even the HW was identical...). Today, HTC has launched identical WP8 phones to what Nokia plans (except the fancy camera) to release.
Clearly they have to compete with Asian manufacturers within the WP same way they would have done in Android camp.
The only "difference" being in WP camp is that it lets Nokia fight to slice in the WP pie (3% of market), instead of fighting for a slice in the 68% pie over in the android side.
The rational decision would have been going OS agnostic and launching both android and WP devices. If you want to be a HW company, don't try to be strategic in OS field.