I feel like people are suspending their reasoning in order to maximally shit on airlines in this thread (because yes, they do have a history of predatory pricing practices). The problem with this isn't the difference in prices - charging less for buying in bulk is a normal thing that's probably been done by merchants since the invention of money. The problem with this is the lack of communication. There's no advertis…
All airline pricing is unadvertised and not communicated. They also sell through lots of independent agencies and channels. It would be weird to specifically advertise this. Unlike say, buying a second pair of shoes - not many people will buy an extra plane ticket to save 30% off both.
That was kind of the premise of a movie I watched last night where a couples retreat offered a group discount for 4 couples or something.
So I could see it being "Bring your friends for 30% off!" being a cool summer promotion to beach destinations or something.