Costco cashiers are under a lot of stress - can lead to rule-enforcing behavior  that seems curt or rude. 

Nonetheless, not letting someone pay with a non-member card is a rule. There is no process for the cashier (or a low-level manager) to skirt the rules, no matter how well-intentioned. 

I don't like that it's so unbending, but it comes from far above the people who live around you who work at your local store. And has nothing to do with Remembrance Day.