> It’s deliberately engineered by the airlines so they can sell that same space twice.
It seems more likely that they chose to leave the existing ambiguity in place than that they deliberately engineered it. They have more to lose than gain by designating explicit right-of-way, same with arm rest possession. Instead they leave it to bottom up negotiation, which works well most of the time.
Also, if everyone on the plane leans back, as most do, it's all the same amount of space, just at a 45° angle. So the airline is just selling a space with the ability to realign itself as needed.
It seems more likely that they chose to leave the existing ambiguity in place than that they deliberately engineered it. They have more to lose than gain by designating explicit right-of-way, same with arm rest possession. Instead they leave it to bottom up negotiation, which works well most of the time.