I don't believe anyone is really letting them do this. They chose to do it, and are now in the process of facing the consequences. I doubt a group of 40-50 teenagers can keep their mouths shut about this for too long.
Truthfully I don't think there is much anyone can reasonably do to "ward off" something like this in the moment. In the same way there isn't much you can reasonably do to stop some one from driving over pedestrians in the crosswalk in front of their car. Most people just choose not to do that kind of thing.
If it became a problem, Wallmart could potentially limit the number of unaccompanied minors allowed in the store at once, or some other inconvenient BS like that. Most people grow out of that kind of crap by the time they turn 20. At this point I think it will be up to Walmart to decide if losing $2000 worth of merchandise this one time is worth imposing onerous restrictions on everyone l like that. According to the police officers doing the investigation they haven't seen anything like this before.
However the bottom line in my mind (maybe not yours) is that I don't believe a single person with a gun could have really helped this situation. In fact I don't believe a single police officer could have stopped things in that moment either. On top of that $2000 in damaged goods in a store probably isn't worth anyone's life. I'm sure Wallmart would rather have the PR that they were the victim of something like this, instead of the story being that Walmart plans to arm loss prevention employees. (Can you imagine how shitty something like that would go?)
After the fact once everyone is separated and has gone home and no longer has "mob muscles" it should be pretty easy for law enforcement to track down the people involved. All you need to do is look at the guy whom they arrested's phone records, figure out who his friends are he was hanging out with that evening, what high school he goes to get out a year book and match the faces to the video. Check call logs, and texts for those people, rinse and repeat.
As a side note $2000 in damages done by 40 people is pretty pathetic. That works out to $50 for each person. I feel like I could do better than that.