Sometimes, the best strategy for survival is to run the hell away. Jack, having lived in the Nevada wasteland for years, was well aquatinted with this fact.
He sees two figures, quite obviously not in the black-clad metal armor of whoever the hell those people are. So he looked at them through his scope, and he saw ray and Fox a little while Down the street he was on. He decided that he might as well join up with them. More numbers means more guns, right?