conventions are organised by volunteers and have had no formal support. If volunteers want to create a convention during their countries spring/summer than there is no rule stating they can not.
There is no such rule, but it would probably not be fair to call it the International BC Anniversary Convention, seeing as it doesn't happen around the anniversary.
And there is a form of support: the BC Supply Store shows up at most Anniversary Conventions and if that can't happen a box of stuff will usually be sent over.
By the way. Bruce made a virtual appearance in Mainz by means of Skype.
It's true that the Conventions are organised by the members. But they're not fully separated from BookCrossing as an organisation, no matter how we define that.