Sky and TalkTalk both rather like having customers, and would be unhappy if any significant number of customers perceived Sky or Talktalk's service as being poor, and chose to move to a different ISP because of it.

UK ISPs do/have used streaming video and buffering (or the absence of it) as a battleground of quality - you see companies making claims of fastest or "most reliable" broadband, and it gets them customers as a result.

Mucking up people's ability to access Netflix would bring no benefit - people would just choose a different ISP.

At least in the UK, where people have that choice. This situation may not apply in other countries.