Eh, they probably aren't that principled, but rather they are just very aware at what their brand is. Costco makes it's money on people thinking the big business loves them as they pay $40 for a bottle of turmeric pills, while the gallon jug of turmeric costs $20.
Meanwhile Costco's entire business is based around negotiating a large profit cut for name brands, and letting the consumer in on a tiny percentage of that profit cut. For most things, Costco isn't even that significant a discount anymore.
There's the famous story of the Founder in a meeting supposedly telling the CEO "If you increase the price of the hot dog I will fucking kill you"
If this actually happened, it's because the CEO knows that the $1.50 the hotdog costs is essential to the brand, and not because he has any belief about how much a hotdog should cost.