The pool of qualified people, for a cashier, is basically everyone.
The pool of qualified people for, say, working at a tech company, is not as diverse [1], and don't match the general population.
[1] https://siliconvalleyindicators.org/data/people/talent-flows...
Edit to add: A better corporate environment, of course, does tend to lead to a better customer experience, but the "visibility of diversity" should not be the goal but rather "genuinely fostering an inclusive environment where people are respected and feel willing to put in their best work," and I think that shows at Costco.
The cost of a Costco membership is $65 per year (really half that if you can share the 2 membership cards you get between two families), available to everyone, and the prices they have there are so good that even my 3-person family saves money each year by shopping there. Every family I know here in my local area shops at Costco, rich or poor, because the prices are so good for many things. I don't see how any of that is exclusionary on racist or classist lines, it seems to me like Costco is one of the good corporations trying to give a good service/product and low prices.
