People who rob travelelrs for their money aren't exactly Robin Hood sorts. They're generally the lowest kind, the type lazy and vicious enough to make a living off taking other people's livelihood away.
They were quite obviously willing to ambush and murder your group, and only hesitated because they were spotted first, you guys have some obvious warrior types complete with full armor and big weapons, and there was a solid intimidation roll done. Plus, you caved in and paid them - not a lot, but enough that actually taking you guys one was definitely the stupider of the two choices (take some money, try to fight you for everything and maybe be one of the people who dies).
This way, they get money (a fair chunk for bandits!) and live until the next group of travellers - probably less well-armed - happens along.
And it's pretty obvious what happens to them. :(
*edit* Even so, I decided to beat that point home with a sledgehammer.