Do not assume that the only reason people return items while claiming a defect is that there is, in fact, a defect.

Yes, if a technically-savvy person tells me "I've done X, Y, and Z, and it still doesn't work", I will believe them. A member of the general public? Even if they aren't scammers, it is entirely possible that they will eat up hours of effort at the store trying to do this.

It's obviously not free. I've seen a low-staff store - I was in a pharmacy (erm, if you're not in the US or Canada, our pharmacies carry a lot more than just health products, though in this case I was there to buy a product you would find in one in the UK or Europe) last week in Canada where they didn't have a cashier at the front. Only the pharmacist and a couple of techs at the back. If you needed assistance rather than self-checkout, you had to ring a bell to summon someone.