The CEO doesn't get involved in the line item details - just announces the high level strategic decisions.
There is a likely some perceived value, to both Microsoft and Nokia, in communicating this strategy to the market/developers/partners - I'm not sure _what_ that value is, mind you, but that would be one reason why the high level deal was announced before all the nitty gritty details were inked.
Interestingly, this may be his specialty: check out his movements preceding the sale of Macromedia to Adobe.
http://www.siliconbeat.com/2008/01/11/microsoft-beware-steph...
...then you make them a high-valued member of your senior leadership team...
http://www.crunchbase.com/person/stephen-elop
...then when opportunity strikes and Nokia tries to buy him away you set up an under-the-table quid-pro-quo deal that would, one presumes, dwarf the $6m signing bonus Nokia gave him:
http://siliconangle.com/blog/2011/03/11/stephen-elop-of-noki...
Sure, it's just conjecture, but it's not implausible.
The far simpler explanation is that if Elop is going to make a big bet with Nokia, he feels safer doing it with MSFT (who need Nokia, and where he knows people) than Google (who don't, and where he doesn't).
Edit to reply below: your probability analysis ignores Elop's improbable behavior as described by hollerith above. And, again, this is not my explanation (this conjecture was all over threads discussing the deal the day it was announced). I'm not even saying that I buy into it, although I do have to acknowledge that people involved with deals in the billions of dollars are seldom strangers to opportunism.
The Nokia board fired the previous CEO because, well, his pet strategy was failing. So some kind of radical shakeup was needed. The two most obvious alternatives were “get in bed with Microsoft” and “get in bed with Google”. While Google doesn’t need Nokia’s help promoting Android, Microsoft desperately needs some major player to support Windows Mobile. Therefore Nokia is in a better negotiating position w.r.t. Microsoft than w.r.t. Google.
In short by kicking up a fuss then they have killed the DOA claims. It means more people will have been looking at the platform. If they had delayed until now more mind share would already be locked into Android or iPhone. Unless a major player made a lot of noise about WP7 nobody would bother learning to develop for it.
Didn't the very public declaration, which IIRC caused resignations by engineers working on Symbian and Meego, radically weaken Nokia's negotiating position?
I must be missing something because it is unlikely that the leadership of a major corporation would be that unskillful at negotiation.