While you can definitely stop a dog from where it wants to go sometimes, I don't think it's always possible nor quite so simple. I used to walk a friend's dog while she was at work, and he would wait to stop until he had to go *right then* (I suspect to draw the walk out as long as possible
). There was basically no time whatsoever between him stopping and, um, the product. Sure, you could tug him away, he wouldn't resist - but it would be mid-poop, not pre-poop.
Of course, even if that's the case, there's no excuse for not having a bag to clean it up.