I think a fork is just an internal branch on the original repositoriy for the Github Servers. From a data / sys admin point of view, it makes sense, since a hard-copy of a repository (and all the git objects with it) is just generally useless.
I would guess that it's stored and dealt with using a basic "copy on write" strategy. It's functionally a copy, which is all that usually matters. This may have been an oversight.
Yeah, I wouldn't be surprised if this was an oversight as well. I can't imagine that GitHub would delete the git blobs from their servers, even if you stop paying (harddrives are cheap!).
