It's to stop people from taking undefended objectives all over the map and getting themselves on the leaderboard. Killing players will always give the most AP, you have to PvP to make any significant AP gains.
Still seems odd to get a base of 800 AP for the Fighter's Guild dailies. Seems like towns' dailies are more in line to the war efforts. It feels more appropriate to offer 25 AP per town daily, than to offer 800 for the Fighter's Guild which has nothing to do with the war. It's just jarring, of all things to offer AP for.