Still, if a few impolite people happen to come at the same time, start arguing with each other, and persist long enough to attract more impolite people from outside, the community is ruined.
Also the norm violators do not need to be consistent. For example they may be polite most of the time towards most members of community, but impolite towards a few selected ‘enemies’. If the rest of community does not punish them for this, then their ‘enemies’ may decide to leave.
Still, if a few impolite people happen to come at the same time, start arguing with each other, and persist long enough to attract more impolite people from outside, the community is ruined.
Also the norm violators do not need to be consistent. For example they may be polite most of the time towards most members of community, but impolite towards a few selected ‘enemies’. If the rest of community does not punish them for this, then their ‘enemies’ may decide to leave.