I agree with almost everything rhollerith said (Most obvious exception: The pain from being kicked is probably more a warning that you're in a situation that caries the danger of major damage than an indication of reduced capacity to take damage in the future.) and would like to point out that the examples I gave are not the only possible ones. In your example, the risk of miscalculating and actually doing long-term damage is relevant, as is the psychological implication of being attacked by a stranger in public. Plus, as I discussed here, enough people have a goal of avoiding pain that you can safely assume that any random stranger has that goal, so inflicting pain on them is harmful in light of that.

Edit: It'd also be harmful to me to go around kicking people in the shins - I'd quickly get a reputation as dangerous, and people would become unwilling to associate with me or help me, and there's a significant chance that I'd wind up jailed, which is definitely harmful.