But since they are both part of BT Group, Plusnet may be offered BT Sport on favourable terms.
That's like saying that the latest whizzy feature on an Audi should also be made available for customers with a VW or Skoda. Just because a company is part of the same group doesn't necessarily imply that there's a business relationship between them. In fact, in many cases, the owner of the group will deliberately set their businesses apart, with different product sets, to target different markets.
I really don't hold with all the 'BT Retail have got BT Sport so why shouldn't Plusnet' arguments. If you want BT Sport, move your service to BT Retail and have done with it!