Most people aren't "stupid". They just don't realize that they need to think about everything they do. People need to be trained. I think all ranges should require everyone to go through a basic common sense/safety class and if they pass they are issued a card. No one should be allowed onto a live public (or private range) without showing their card. I'm sure some will strongly disagree with this idea but in USPSA competition everyone is checked out before they're allowed to shoot. It prevents stuff like the OP saw from happening. Just letting anyone step to a line with firearms would be like going to a motorcycle dealership and taking a new bike out when they have never ridden before and don't even have a motorcycle endorsement on their license.
I've only met a few dealers that were concerned about knowing about a motorcycle endorsement while making a sale. Met more people doing private sales that are more concerned with permits or endorsements.
As for the OP, probably best off leaving since you had your kids with you but if you didn't...you could have had a teaching moment if the guy really didn't know anything about his gun. Could have reminded him of general range safety (maybe he is new to shooting) and given him some pointers so he doesn't make the same mistakes again.