Cheap innertube use it or trash it? 

Use it until it goes bad. It may last a long time.🤷‍♂

could be a cost saving measure or could be a structural integrity measure extruding a single tube insures that the wall thickness is consistent for the entire innertube but it would obviously have to stretch more meaning it’s under higher tensile stress, so who knows

As long as it holds air. The tire is the part that has to be thick. When it goes bad, it can be used to wrap a saddle horn for calf roping.