In general it is possible for an installation to make certain system changes other than the record of files written during an installation (for example, under Windows, there may be distribution-specific Windows registry updates).
There seems to be no hook that's called when a distribution is removed, which would allow distribution-specific cleanup code to be called (e.g. to undo Windows registry changes).
Such a hook should be provided, and I've created a ticket for this:
I believe there should be a pre- and a post- hook, just as there is for commands. Éric has suggested getting opinions from the mailing list, so what do you all think about this?