There are two ways to get the passing shot, I think: One, lots of later units have the "zone of control" ability which gives them free shots on units moving past them (cavalry is the earliest ZOC unit, I think), and two, I'm pretty sure that any unit in a fortress automatically gets this ability.
Back to the topic, yes, I have seen the AI plant fortresses; usually on resources, but sometimes on chokepoints or hotly contested borders. If the AI built more workers, perhaps you'd see more.