There is too much organization involved in freeing them all and all it takes is one of them, or one of our group, to be detected and the alarm sounded. The odds of going covertly with what seems to be a really disorganized camp, attempting to steal or use the horses in the process, is safer. While we might want to save them, we fail and die then no one saves them. We can kill bands of them or hunting parties, but the entire army as it comes to stand to? A few too many.