As far as I understand the circumstances, because Microsoft and Activision-Blizzard are both US companies, they ultimately fall under US regulation except for any of their offices/holdings in other countries, where they have to abide by the local laws. The reason the FTC is upset now is that Microsoft had said that Activision-Blizzard was largely going to be its own independent company under the Microsoft brand, so these layoffs go against those promises - especially with the wording about removing "overlap" between the companies, which points to them firing people at Activision-Blizzard who had the same job as people already working at Microsoft. The only reason that they'd do that is if they're not actually letting Activision-Blizzard run on their own and are going to be merging the company into Microsoft more than they had said they would.

I do remember something about the UK signing off on the merger, so I assume that there are some countries that did their own "due diligence" and approved the acquisition, but a majority of these layoffs are in California by the sounds of it, so all any of them could really do at this point is hold Microsoft liable if they don't follow local labor laws about severances and the like. I assume that they felt the same way as the FTC, in that the promise of Activision-Blizzard running on their own meant that there was little concern about monopolizing the industry.