Actually, they do. But as time goes on, the subscription's stipends value decreases as subscribers get the items they want.
Basically
Subscriber receive a CC stipend which is best spent on f2p unlocks
Which are sold on the GTN for 350k to preferred players
Sort of a trickle down effect.
The main problem with this model is that the more you improve your f2p account the harder it is to switch to subscriber as it would nullify your efforts.