I don’t know about you, but for me Tisha B’av is a meaningful, yet tiring experience. We need to rest up for tomorrow, and cook a meal for tonight 🙂 I think it’s very reasonable to end at 12 today.
Because they probably learn Toroh their entire time at camp as well as when traveling home as per “uVeLechtecho baDerech” and couldn’t learn today after Chatzos, so they had to factor in enough time to still be able to learn Toroh whilst commuting home.