I'm with the other comments: where do they say they cap at 12%? Costco brings a lot of value to the consumer, IMO, but I feel like that's deliberately shooting

I'm with the other comments: where do they say they cap at 12%? Costco brings a lot of value to the consumer, IMO, but I feel like that's deliberately shooting themselves in the foot. And why the arbitrary 12%? Why not 15 or 21?