Unbelievable. I will be torrenting like crazy. Never going to accept limitations like this.
If you can - and will - torrent, why buy even if you could download?
I'm guessing it's down to storage and user experience management. A 4K movie can easily reach around 35GB+ so will eat up around half the space already of a 64GB Apple TV.
My entire iTunes purchased library is stored on a Drobo 5D - I stream from it to my AppleTV.