>What I don't understand about that strategy is what prevents the acquired employees from re-forming as a new company?

Nothing prevents this, there had been many studios by former employees of a shut down studio (e.g. Ensemble -> Robot, Pandemic -> Killzone, Incognito -> Lightbox, Big Huge Games -> Impossible etc. etc). However, usually the reason a studio had been shut down was its poor performance and not some evil marketing plan so the respawned studios inherit the problems of the original studio complicated with the talent drain that happens during such a transition.