It's called married segment logic. Technically the multi-city option means you're booking to fly from A-B and then B-C. Even if it's the same day it's two different flights you're booking. On the RT you're actually booking A-C just connecting in B. At least for now DL is still allowing you to fly on tickets booked like above without re-checking luggage, getting protection on continuing flights, etc. However they're looking for ways to close it. The US airlines got some bad PR earlier this year. They started making you book into full Y when you did the multi-city approach. They've had to back down, but I'd say it's only time till they figure out a new way to block us from getting around their pricing controls.