I find it nice that they do it - one of the things they check for is items that you purchase via cardboard card (laptops, gift cards, gym memberships, etc...). I'm always surprised that people find it so offensive that someone is double checking that you got all your items while double checking that you aren't stealing. I've never once felt like a criminal. Proud member since 2001.

In most (every?) states it's illegal for them to force you to stop. I don't have a Costco membership, but some other stores around here pull the same stuff sometimes. I just politely tell the person, "No thanks" and keep walking. With Costco you risk them canceling your membership, so take that path with that risk in mind.

the flip side of this is to ensure you actually have all the things you paid for.

if you paid for 6 loaves of bread, but you only have 4 in your cart that's also a problem.

I've had the door checkers catch double charges on my receipt twice (!) in the last decade or so. They're honestly a net benefit for me so far.

Well, yes, that's the intent. That combined with their members-only policy results in significantly reduced shrinkage, which further allows them to drive down operating costs and thus prices.