The "ByteString" in the SYstem.Posix.*.ByteString modules applies to the pathnames (POSIX pathnames are byte strings, and there's lots of ways to cause problems if you insist on pretending that they are in any particular encoding), not the values being read/written, so functions like that are basically carried along just to provide a mostly compatible API.

I would expect to find functions working with ByteString values under Data.ByteString.