They don't restrict anybody. BUT, you do need to sign up for a membership, which costs a little money. In reality most people make that back in gas savings , $1.50 hot dogs and $5 rotisserie chickens.

Aside from the benefits people have already mentioned, in the current climate, having people not able to anonymously come into your store and steal things is a great benefit for them. I would imagine their theft rate is at least an order of magnitude below other big box stores. They treat their employees and customers well, and because of that the customers and employees treat them well in return.