Pretty cheap as far as PR schemes go. Letting his children leave and speak openly to the free media would be a PR disaster. Killing them would be a worse one. But keep them where they are and under threat of death while giving the appearance of comfortable, consensual living, and everybody wins, right? It at least offers sufficient plausible deniability for Pompeo to continue to ignore the SA government's historical and ongoing affronts to US sovereignty, which is all anyone with a real hand in the stakes cares about.