My mother spent a couple of years in bank teller training, and I've taken discreet math classes. Accordingly, I cross through empty spots in forms and write my t's with very big tails. And when I deal with physical paper, reports and tickets or coupons, once invalid I rip them.

GAs and others should not trust a physical or electronic boarding pass (but often do... Lounges...), but validate them against the computer record. So why voiding them by ripping them up?

First, habit, knowing or being trained by someone who grew up when "do not fold, spindle or mutilate" meant something.
But second and most important, cogitative load. Have you seen behind a typical podium when it's busy? Scrooge McDuck could swim in the piles of paper they have. An intact paper BP or app screenshot are suspicious, but a ripped one is unquestionably not worth considering.
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An overt and almost ceremonial destruction is clarity for all and harkens back to a different time. It's no more violent than calling someone who works for a living "Sir".
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