Actually, one of the things in the agreement is that Windows Phone will be Nokia's "primary smartphone platform", so there probably is some sort of legal bind, although I would really like to see Microsoft suing Nokia because Harmattan consistently outsells WP platform.
But I think the bigger problem is that Elop's actions are pushing Nokia to the point where they will not be able to support own platform due to lack of expertize, people and services even if they wanted to.