It's probably due to licensing. These are all Netflix originals where as movies or partnerships would probably charge a fee for offline content. I'd imagine offline would be considered "rentals" so it would be treated similarly to Amazon Video or iTunes.
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Seems like only a limited selection is available offline.
From the article: "For example, Orange is The New Black, Narcos and The Crown are available for download today."
It's probably due to licensing. These are all Netflix originals where as movies or partnerships would probably charge a fee for offline content. I'd imagine offline would be considered "rentals" so it would be treated similarly to Amazon Video or iTunes.
There are some non-original content movies and such too.