Vivendi sold them to Activision. Blizzard itself had no say in the matter. Blizzard hasn't been its own master since soon after it was formed.

This is pedantic but didn't Activision buy Vivendi's game subsidiary completely? I assume Blizzard was the target asset there but I seem to remember that the whole company was purchased.

To be even more pedantic: there was a deal where Vivendi's games properties was merged into Activision, the combined entity was renamed Activision-Blizzard, and Vivendi got controlling interest in it.

Oh interesting. I didn't realize Vivendi was still involved. I thought they sold the games department to exit the industry but I guess it was the opposite.

That deal was in 2008. Five years later, in 2013, Kotick raised money to buy out Vivendi, and Vivendi exited the industry shortly thereafter.