What level of injury to the man on the ground is acceptable, then? Bruising? Broken bones? Brain damage? Spinal damage? Never being able to walk again? Never being able to feed himself?
How are you going to ensure that, wherever your line of acceptability is, it doesn’t get crossed? After all, a pile of people coming in and just kicking wherever they can kick have no way of really knowing what the consequence will end up being.
Perhaps it could be incumbent for all would-be smash and grab thieves to adhere to some possible future guidelines of health and safety, including protective headwear, knee and shin pads etc?