Listing and approvals, (labels) from an NRTL are not on all items here in the US. Basically, the AHJ has the decision to reject any 'non-labeled' items.
In a perfect world, there would be UL?CSA (or other approved NRTL) labels, on everything, but we are far from a perfect world.
With safety related issues to any unlisted item(s) we have various government agencies that may persue the item & mfg. UL also may investigate items that pose a safety concern.
As to the stage equipment; the responsibility of 'checking' has to fall on the venue management.