I think the revenge isn't about the cashier. The old lady found a loophole that got her what she wanted, in s way the system provides the cashier to comply with. The revenge is about the system

The cashier's getting paid the same. If they were like waitstaff that get paid by the customer, it'd be a shitty thing to do, but for folk that are hourly/waged, it's still not nice, but not quite shitty territory, since you're not messing with their income, just time (and as someone that's worked customer support, I'd rather count pennies for 2 hours than deal with another irate "I'm angry because I can't read" ahole)

I love simping for shitty rules

Yes, hassling the employee is fun isn't it? Asshole.