If an AI can be built, it has been shown that humanity is an AI-creating species. As technology and the promulgation of human knowledge improves, it will become easier and easier to make AIs, and the risk of creating a UFAI that the FAI can't defeat goes up.

I would think that something as much more intelligent than humans as the FAI would be able to prevent humans from creating an UFAI that could defeat it without limiting their intelligence.