Since the argument was made above that a 40' roll away should have been an 80' roll away, we're not talking just inside the circle.
Hence, if you want to hold player B responsible (i.e. penalize) when player A's disc hits their bag (or person), then you need to consider the case where player A rips an approach shot that careens off a tree.
A worst case scenario is that player A might move his bag and stop the roll away, but argue he was trying to avoid the disc.
Just too many exceptions to go away from the existing rule which is basically that *if* the field remains the same from the time the disc legally leaves the player's hand until the disc stops, no infraction will occur.
That wasn't an argument, I was being facetious. I like the rule as is.