I'm sorry but this is the behavior of a dying company grasping for remnants of how things use to be. I personally HATE this nonsense about having to use all of the legs.
Every airline in the US follows this same practice, so the 'nonsense' apparently is important to revenue management.
Even from this idiot's chair, this isn't difficult to understand why hidden city ticketing is prohibited by ALL airlines.
All together now...airfares are not driven by the cost basis....