I’m doing the work of an employee to scan and bag my groceries. Stealing almonds is the wages I am owed for my service.
What would you say of the case where you do all your grocery shopping and they close the full service checkouts right as you wrap up and force self checkout? You’ve agreed to shop with the intention of having a cashier and they unilaterally change the terms after you’ve invested your time… I’d say unilaterally changing goods received is just as much theft as unilaterally changing payment.
(Which will usually actually cost them more than just stealing some small item which doesn’t justify either.)