I am also at a loss as to why customers seem to think unloading is for the cashier to do? Unless the counter is set up that way.
If there's a misunderstanding, I've always found it works best to ask cheerfully
Most customers don't mean bad, they just have no idea how it works at the till.
For example, our tills are set up so we load groceries from one cart into the next. We're supposed to work front to back, back to front, so you work sideways, with absolutely no bending over. Sometimes a customer has only a few items, so they're in the back of their cart, which means I am putting them in the front of the next cart. There's a lot of customers who proudly put their bags in the back of the next cart. I usually pull them closer, but it's really mindnumbing how many pull it right back
Again, cheerfully explaining what you're doing helps.
Still, there's customers who will always take offense no matter what. The carts have two levels, and we always put the bottom stuff in the bottom of the other cart. Again working sideways. I once got completely chewed out because I refused to move heavy items top to bottom. Patiently explaining that it was company policy to safeguard our backs did nothing, because I was obviously young and they had no such nonsense 'back in his days'.
As our bosses put it: the customer is there for five minutes. You are here for life. Your health is important. At my job, the cashier is 'boss' at his/her till. While remaining polite and explaining what you do, of course. But nine times out of ten, politely stating 'I am moving your bag so I can reach easier' and 'I am going to move the cart back a bit' gets absolutely no fuss for me. The other one out of ten was grumpy when entering the store, and will probably still be grumpy by the end of the day, so don't take it personal
My suggestion would be: ask your manager what the policy is, and what he suggests you do in such a scenario. If he tells you to reach up, I would personally refuse. That is hell on your body and will destroy you shoulder in a couple of years.
I vote not rude of the customer though (some genuinely don't know, others zone out), unless they make a fuss once you ask them to unload.