I don't think I would do anything with those two guys either, but I wouldn't let them continue carrying. They would gave to atleast lock their guns in the trunk or something similar.
There's just too much liability. What if these guys drive away and get into a shooting somewhere? Questionable or not, you may get sued by both the victim and suspect. It's not all that different than allowing someone who is intoxicated to drive off because they were right at the threshold.
Sadly enough your right on the liability part. Not sure why legislature hasn't been passed allowing on person or in car when traveling through. Of course not all states require the same amount of background searches and training to obtain a permit.