Is it peculiar for a department not to have any of its own PhD graduates as professors?
It's common in computer science for a university to have few if any of its own graduates as faculty, and should not be taken as a bad sign by itself. This low representation is for two main reasons:


The best five or ten universities in the United States produce a disproportionately large number of strong academic candidates. In practice universities that have research departments at all (let alone the exceptionally strong ones) tend to be filled with professors with Ph.D.s from the elite few.
Going somewhere else leads to cross-fertilization of academic thought. Presumably after 5+ years of graduate school one has spent a lot of time learning from the expertise available in their department. Going somewhere else allows a professor to transfer her knowledge to a different group of researchers, and to get fresh insight from them in return.