It's easier to use the same disc for the BD and 4K+BD releases instead of making 2 different discs and still stay within some of the legal requirements when studios don't own the rights to items in all countries.
It all gets back to some of the reasons for region locking discs at all, but until DVD and BD don't exist any longer I'm sure they have to follow the established (or at least accepted) legal boundaries for a format
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