I am not an expert in this as it has been so many years since I worked in nat. gas. but it I seem to recall that pipelines (at least the ones I worked on) did not have major storage. Instead, they simply renominated volumes on the pipe itself (more of a swap with another player) and called it stored gas (for a fee of course).
Knowing this, I would say the interuptible customers are getting hosed for space so that firm customers can use it to move their gas since they pay more for the transportation.
What I don't know in this equasion is: is this like the airlines business who over book every flight hoping that someone does not show? If this is the case then it may be said that too much gas is floating around looking for storage.