Are you deliberately acting obtuse?
Just because you have a PS3 does not entitle you to play PS3 games on your PS4 (at least, as far as we know).
Similarly, owning a 360 does not entitle you to play 360 games on the Durango (at least, as far as we know).
This rumour indicates that they're offering a cheap stand alone device, that - in addition to its media capabilities - can be connected to the Durango to enable BC.
Essentially, without pricing themselves out of the market by including BC by default. They have given the consumer more options.
Bizarro world indeed.
In Bizarro world it makes sense to buy a BC device when you already own a 360. If you don't already own one why would you want BC?