Many [Nation] Servers have it as part of their rule set. But mainly it just étiquette to make sure all players have good experience. The [Christian] Faction have a rule about that in their manual. But they don't really stick to it in PvP situations.

Legacy rules from an older build.

Many players, including me, have an inbuilt constrain system on our behaviour, not that we can't do something, but that we don't want to. Some players, sometimes tagged as 'psychopaths' or 'sociopaths', don't have these constraints. Some players think that the reason most players have it is that in an earlier version of the game where most players were in small clans, acting against the clan's interest was good for the individual player, but bad for the group, so gradually it was bred out of these players to act selfishly, and the 'empathy' trait was passed down from players that had it, to their spawn, and this players that had it made better groups, so those groups survived.