I'm wondering if this is an issue of seating capacity, or of maneuvering-time when unloading.
Has anyone observed how people with open single strollers are tending to move them off the gondolas? Are they turning them inside the gondola cabin and rolling off forward, or are they backing them out?
If they are backing out, and the folks in charge of load practice observation are seeing slowdowns and/or fall potential in that practice, that would explain requiring doubles to be folded. Doubles are mostly too wide to turn inside the cabin without being able to lift a bench, and the door clearance is rather tight for backing out, which makes the process a tad slower.