This is so that the interwiki and interlanguage links show up on the translated subpages as well.
Additionally, the Translate extension should probably lie to Wikibase about the existence of translated subpages, so that when Some page is marked for translation, then adding Some page/en, or some other translated subpage, as a sitelink on Wikibase would be disallowed, as if the translated subpage didn’t exist.