Even if ALL the bags go through the x-ray machine, I have a point. The x-ray machines don't work alone. If the machine can handle 60 bags per minute, you'd need an extra minute of staffing for every 60 extra bags (and, you'd need more than one person working, so really it would be an extra 2-3 minutes of staffing). Okay, 2-3 minutes of staffing isn't much but I suspect that the machines do not handle 60 bags per minute (that is one second per bag!) and there are a lot more 60 extra bags per day.
Now, if we had automated x-ray machines, that could be used in absence of any workers, I wouldn't have a point.