Approval by the European Commission was necessary because Vivendi (the owner of Blizzard and now the buyer of Activision, if you haven't been keeping up with all this) is a French media company, and therefore subject to EU business laws and antitrust concerns. Officials were mulling over the merger because of fears that Vivendi's ownership of Universal Music Group would give Activision Blizzard an unfair advantage in licensing music for games like Guitar Hero.
They finally decided that it's not a threat to the health of the market, and approved the merger. So there it is. It's done. The government can't save you now; Activision Blizzard is your new master. I tremble in terror before the fictional (yet somehow inevitable) Bard class and its l33t Guitar Hero skillz!