You would never expect to find Mercedes ads on BMW's website. Also everybody knows that u don't need to look for Windows advertisments on apple.com. So I think it is also quite normal, that Facebook doesn't want to advertise G+ on their website.
Indeed. But on the other hand I don't think Google would disallow or block ads for Facebook. Their strategies differ. Google thrives with an open web, Facebook is a walled garden. But yeah, no scandal here.
[1] AdWords - Wikipedia - http://en.wikipedia.org/wiki/AdWords