Costco is a membership store so they have the right to condition your membership on showing the receipt at the door, but at Fry's you can walk right past the do

Costco is a membership store so they have the right to condition your membership on showing the receipt at the door, but at Fry's you can walk right past the door checker if you wish and they have no right to detain you as long as you haven't done anything illegal (like steal an item) since private citizens (like store security) has no right to detain you.