The idea of no inventory applies to revenue tickets as well. Often times United, for example, will zero out a certain sector so the the entire RT cannot be booked.

This works really well to 'land locked' destinations (ie, OW in and OW out).

For example, a while back UA had a really cheap fare to SIN. But they zeroed out NRT-SIN and HKG-SIN so that no seats for that fare were bookable on those portion. One would find inventory on all other sectors.

United continued to advertise the low fare - and then bait and switch.

One of United's newest techniques is to zero out L seat fares out of the hub. Often one will find L seats available on the first flight in and the last flight out, but no inventory available on the connecting trips. It's designed so that one 'misconnects' with L inventory - so one cant 'move on' thru the system. And it prevents mileage runners from adding segments.

Thus, inventory is controlled.