Technically it's to provide an opt-out switch in the interface. It's being developped and will land in the stable.

This shouldn't be an opt out, it should be the default. Most won't understand the ramifications of not opting out.

Breaks the distribution and cost model. You either need to join the swarm or pay to watch the video from a CDN. There is a cost to deliver bits, it's just been hidden from you by corporations subsidizing it. Therefore, it can't be default (although a modal could be provided that informed you prior to streaming and gave you the chance to bail out).