Nowhere in the article I see disrespectful treatment of the Dad. If dad went to the store by himself this wouldn't have happened. 
I see a spoiled daughter who thinks she can use her Dad's vet status to bend rules. And who has a meltdown when she fails.
She created the situation, not Costco:
- by going shopping on Remembrance day right before the ceremony
- by adding her own shopping to the cart, and refusing to drop them so her dad can pay cash for his own items
- by rejecting staff's solution to get her own membership

The "vitriolic" comments are common sense solutions to a situation that shouldn't have been escalated in the first place.