If you bought it, it's up to you to be able to prove you bought it. It might not be the best analogy but if I bought a tin of beans, then took them back for a refund it would be up to me to prove I'd bought them from the shop I took them to (by receipt) not the shop staff to prove I bought them there. (staff view and passenger)
The 'problem' there is that the larger supermarkets can afford to take your word for something, and just swap out or refund you without a receipt. It clearly pays off for them in the long run, but does perhaps make people expect everyone to work that way.
Sadly, there are too many people who are trying it on and the genuine, honest, people suffer. That's why I think the industry should have an appeals process that takes place later (so staff aren't in such an awkward situation) with an admin fee. If you're genuine, you'll be okay (less the fee) but if you were trying it on then you'll get nowhere without engaging in some seriously fraudulent activity that could (no, should) land you in deep water.
