In this specific case, Fitness+ is relatively cheap to produce (all the content is theirs except for the music, but they already have a good relationship with music producers) and cheap to distribute. They make 1-3 new videos per instructor a week. I doubt it's costing them much per user to distribute the content. However, the Apple Watch is a nice profitable device for them and Fitness+ requires Apple Watch. So for anyone who already has the highest tier subscription (probably a decent chunk of multi-iDevice families out there) they will find themselves having Fitness+ but being unable to use it, which creates one extra motivator to buy an Apple Watch.

That's where they end up making money anyways, 75% or so of their revenue is from device sales (computers, phones, tablets, watches, etc.). The services help to keep people within that hardware ecosystem which gets people onto the upgrade cycle or to consider a second or third Apple device instead of just having one.