Hello Archon. It was the question of a lot of players, but the reply is obvious, to prevent the bigshot players to force the smaller players to quit the game. It is so since this game started, and it was fairly balanced to protect the smaller players. Keep in mind that the besieged player cannot rebuild the traps, and he or she loses a part of the city's resources. I do hope that it will never be changed, as there are always bigger players than us in the game, so we all might be a  possible target to be besieged. 🙂