This sort of thing has been happening for years. When Canadian airlines was around it was often dramatically cheaper to fly SEA <-> YVR <-> HKG than it was to fly YVR <-> HKG. I used to have friends flying to Hong Kong out of Vancouver who would do exactly as you describe - Make their way to SEA and depart from there. Often they would fly with carry-on and on the way home they'd just walk off the plane in Vancouver

It's just he nature of the beast when it comes to airline competition.