Assignment #4 - In which I am baffled

The fourth rule that Twain accuses Cooper of breaking is as follows:

"They require that the personages in a tale, both dead and alive, shall exhibit a sufficient excuse for being there."

Sufficient excuse for being there. As in they have a plausible reason for doing what they're doing, or going where they're going. In The Last of the Mohicans, Cooper breaks this rule right off the bat. What is Cora and Alice's excuse for travelling with the army reenforcement? To join up with their father is what we're told, but what would possess them to join their father in a fort that they know is going to be under attack in the very near future? Just because it gets the plot moving?

Almost immediately after the two girls - one of whom, it should be noted, is just a terribly delicate flower - decide and are permitted to travel with the soldiers to a fort that will soon be under seige, they manage to make an even more baffling decision. Instead of travelling with the column of 1500 soldiers, a force that could be considered somewhat unassailable given the circumstances,  they decide to break off into a small group of four and travel by a "secret route", led by a man who was once an enemy of their father. And this is before they decide to include a singing madman into their company.

In Magua's case, his reasons for going along with the entire ordeal are easily explained as they only make his job easier. But from the point of view of Alice, Cora and Heyward? This all happens only because it has to. Because the two girls have to be separated from the group in order to meet Hawkeye and the Mohicans. And having something happen simply because it has to, while paying no heed to how it happens - and weather it is organic or not - violates Twain's rule #16 - "Avoid slovenliness of form".