Do they give you a discount because it's non profit? I know background checks for teachers typically run $40 or so. Personally, if I was running a show like this, I wouldn't want to have the liability issue of trying to ensure that all vendors there are "safe". Just because the vendors all pass background checks doesn't mean all patrons would. Plus, would everyone who is working the show need a background check? For some small businesses, this could be cost prohibitive. And as
@kschweer said, it takes time to get those in. What happens if one employee gets sick and another is going to fill in for them?
IMO, I think it is better to operate under the assumption that there will be child predators there, whether as vendors or patrons - or even staff of the actual facility. Now, how do you prevent their access to children or leaving with a child.