This is my personal opinion on the matter. Some take the word "Single" literally. If a group of people that know each other are okay with riding separate with a stranger, and most likely different trains, I don't see any park rules being broken by using the single rider lane.
I am pretty sure it is an option for a few reasons. This technically makes load times quicker, for the ride ops are not spending time latching seat belts around restrains, and helps with capacity.