I'm pretty sure I've seen this restriction discussed on HN before, especially around the time g+ was introduced. I think they ran into this well, but I'm not sure what the resolution was.
IIRC This was Google's rationale in disallowing FB from importing contacts programmaticaly - FB wants to hoover contacts from everywhere but refuses to give data back.
Correct. That is pretty much exact reproduction of Google's statement when they cut off Facebook access to Gmail contacts. The irony.