I think people should be forced to PvP in a way that I approve and all other forms of PvP should be fruitless. /s
Well the issue is that taking an undefended keep is no player versus player interaction. It´s player versus environment (namely npcs and keep doors).
Your argument is invalid as it´s no other form of pvp in the first place.
You may want to do nothing but directly fight other players in small battles and that's fine. Some people want to put a lot of pressure on the map by taking over strategic keeps, cutting off transit routs, helping their alliance push emp or dethrone an existing emp. Some want huge zergfests. Some want to go around and solo resources. Others just want to come in, get some AP and get out. Players can also go into Cyrodiil and deliberately attempt to avoid pvp encounters all together (skyshard hunting, delves, quests). If people want to PvD (player vs door) then they should be able to do so and others are welcome to come and kill them. There's nothing wrong with doing any of these things.
The game offers other places to go and experience small scale battles outside of Cyrodiil (battle grounds, IC, dueling).
That way, you don't penalize people who enjoy taking keeps with a strategic goal in mind, or even people who enjoy zergfest, but you completely remove the AP incentive from people who play PvDoor when zero opposition is online.