Following your logic, the next time I see a crime being committed...a mugging or perhaps a rape...I'll just keep walking and ignore it since I'm not sure "...whether or not it is legal for me to interfere".
I generally don't carry five sets of handcuffs and go tooling around in the middle of the desert hoping a rape will happen, though. I'm thinking these guys were looking for an instance of border-incursion. Again, I'm not against that, I just think if you're obviously on the lookout for this type of crime, and go to the trouble of equipping yourself to handle it-- you might want to stop and consider whether or not you're going to get thrown in jail afterward.