There's a difference between PK and Harassment. This rule is to prevent a scenario like this:
Guild A is leveling at Dark Bear and is PKed my Guild B.
Guild A resurrects and goes to VD to level and is again PKed by the same players from Guild B.
Guild A resurrects again and goes to level at Cyclops and is PKed by the same players from Guild B for a third time.
That is harassment. If you want to fight over a leveling spot, then go ahead, you won't get penalized for that. You can PK all day to protect your leveling area, that's perfectly fine. However, just going to various leveling spots, killing anyone who is there, and then going to other spots and doing the same, or chasing the same players all over the server and PKing them over and over is harassment which is what the GMs want to prevent.