Again the discussion is moot and I haven't seen where MS has officially
published anything.
But at a guess, the GC partitions are not writable. Or to put it better, you may
find a way to write to the GC partitions, but you can't get that to replicate
into a writeable partition on a DC that hosts the domain.
A better question would be, why doesn't it apply to the config and schema
partitions.