Yes, I've been thinking about that also, and as you may have read in the news in these days, a more exclusive agreement has been considered since the day of the announcement, and it seems it still is a possibility.

Personally, I see your point but I am not sure yet it is really needed: unlike other hardware manufacturers, Nokia has the license to customize WP7. Actually, they claim they'll be working in partnership with MS to that purpose. So, I'm aready assuming that the WP7 OS we'll find on Nokia phones will be to some degree different (albait compatible, I really hope) from the one on other hardware producers. And yes, I too agree that the latter may quite dislike this.
I also think it will be quite likely we'll see other features I can't really understand why are currently missing in WP7, such as thetering and Sync with Outlook.
In other words, I think this degree of exclusivity may be enough to generate that uniqueness that is indeed needed to compete against the iPhone.