What's in it for Nokia with carrier exclusivity?

They MUST know that carrier exclusivity can never sell as many devices as having it available on every network that is willing to take it.

So why did they do it? There must be reasons and benefits for Nokia to do this. Seeing as their current situation is precarious at best, I would have reckoned they would go all out with spreading the Lumia.

Thoughts? Opinions? KNOWLEDGE? (Tom?)