It might have to do with the unionisation, but I wouldn't be surprised that its just that Blizzard is like one of the like 4 money makers that MS still has in the gaming division and that is why they were spared.
Unions also many times (especially with "guild" type unions) can serve other valuable functions like guaranteeing a higher minimum quality of work (generally).
I know it's cliche, but what are the thousands of people that work there doing all day?