In the supermarket where I work, at the smaller tills we unload from the baskets that the customer puts down on the counter. However, there are niches in the counter that the basket sits in, making it easy to reach in and pick stuff up. Once the basket in empty we pick up the basket and place it behind the counter in a kind of trolley until there are enough to put back at the shop entrance. We prefer customers not to empty the baskets themselves, as then they're left holding an empty basket with nowhere to put it. On occasions when a customer doesn't have a basket, just a few items, they put them down in the niche. I'll ask them push anything heavy towards me if it's out of reach, I've only had one person object to pushing anything towards me, and he was a bacon fed knave, so I'm happy to disregard his opinion.
So, no, it isn't rude for customers not to unload baskets themselves, especially if there's nowhere visable for them to put the empty basket. However, if you're having problems reaching into the baskets, that's an issue in it's own right. Tills get designed for the average, and if you're short, you should ask your manager about adjustments, like the step someone mentioned upstream. I have had shoulder problems that required treatment from a physiotherapist, and I think part of that was due to work at the tills.