That's obviously not a blanket case of "AMFA bag handlers categorically refuse to use bag scanner tech" but rather "SW, unlike other airlines, didn't negotiate (early enough) the use of bag scanners in its contract with AMFA bag handlers."

SW decides what their priorities are when negotiating these contracts and deciding the terms. Other airlines obviously prioritized bag scanner tech and made the necessary tradeoffs to make it happen while SW didn't. Half the blame at the very least should be apportioned to SW.