and stuffing the punctured one in your back pocket for repair when you get back home.
I am not supporting what they do but explaining it. I imagine that the sort of rider who commits this act does not have a pannier or saddlebag and they leave the house with bulging jersey pockets. A used innertube is not as tightly packed as a new one (without some effort) and dirty.
Would you really sling an inner tube "bandoleer style" over a white Rapha jacket?
On group rides I have been known to ask riders if they intend repairing their tubes and offering to take it away for them if they dont. Some are genuinely very surprised at the idea that I actually intend repairing it for reuse.