I take the more charitable perspective that the receipt checkers are validating the cashiers' work, and that it is part of the "membership" bargain.

I worked at costco during college. The receipt checking stops a significant amount of shrinkage. At these warehouse stores it's surprisingly easy to walk out the door with large items you wouldn't be able to take at places like Best Buy. We caught people stealing washing machines, tvs, 4-packs of tires, etc. It helps keep costs down to have a quick verification check.

As a member I love it. I want it to be as hard as possible to steal from them, because ultimately I will be paying the cost for theft from them, so it saves me money.

I mostly agree, but my personal pet peeve are stores that treat small backpacks as somehow distinct and more of an indicator of theft than giant purses.

If you want to ban bags, do like, say, the Pearl Harbor Memorial and explicitly ban any bags over some certain set of dimensions.

I’ve never had a problem bringing a shoulder bag or small backpack into a store. I’ll infrequently ride a bicycle to Target, Home Depot, Walmart, a gas station or the local grocery store and put everything in my bag after I check out.

At stores with self checkouts, I don’t really see the difference because they’re already placing trust in you anyways.

I don't get it often, but my local Costco had an overzealous receipt checker who'd always ask to check my visibly near-empty small backpack.

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.

They do a simple count of items you have against what's on the receipt. If the count is off, I may have left something at the register. As well, if I have something large they offer assistance to get it loaded. I have no issues with this check and find it helpful.

And yet you like sneaking into a members only club.