They must intend to use BT Sport to raise average revenue per user, decrease churn by locking customers into additional minimum contract periods and win customers from their competitors.
Of course that is what they are doing, its what most people in business do, offer extras to keep the customers with you and take from other competitors, its nothing new. Its only a surprise to you now probably because its taken so long.
For BT to survive its not enough to just offer phone and broadband because its rivals (Sky & Virgin) offer TV also, they started slowly with Vision (bit pants) and now its growing and they are doing what they can to grow this new venture for them.
Its all perfectly normal and no surprise at all. Tesco start doing on-line food shopping for home delivery as an added extra, ASDA do the same , so do Sainsburys... its happens in all business, nothing new or shocking to see here