Originally Posted by
chollie
At some checkpoints, the bin thing actually makes sense - it keeps dangling pack straps from getting caught in the rollers (if any). For a screener, it's probably easier to just put all packs in bins than to check to see if there are dangling straps. That same screener will (or should) be eyeing the pax-loaded bins as they go through to make sure there's nothing to dangle and snag in the rollers.
In addition to your point about straps, some modern x-ray's user manuals specifically stipulate that specific trays must be used and items to be screened should not just be placed on the conveyor.