It's definitely the motivating driver but from the passenger's point of view, not an inevitability. It's fully possible to be booked on either one ticket using two different airlines that don't necessarily include CA. It is also possible to work this out on two separate tickets with PEK as an endpoint of each ticket. Again, CA may or may not be included as a carrier on any specific itinerary.

Sample: LAX-ICN-PEK and return on OZ. PEK-BKK-PEK on TG. Or as anacapa suggested above, PEK in one direction and PVG the other (though I think Beijing still needs more than a single 72 hours (minus unuseable time, likely one would get 2 full days out of the 72 hours which isn't a lot for BJ.)