Maybe, but they're paying attention to the task of scanning items, running the register, and the customer at the front of the line. I don't really think it's reasonable to expect them to keep an eye on that moving target as well. I've seen the very thing happen: Loading my stuff on to the belt, trying to leave a space because there is no divider available, the cashier is busy concentrating on the other things they are doing and the customer in front of them (not me and my stuff), they grab the last item of the other person's stuff, scan it and bag it, turn back to check for more stuff (by this time and while the cashier's back is turned the void I'd left is gone because the belt doesn't stop advancing until a divider or product blocks the sensor). They may not ever see a gap (only the next item to be scanned).

There's no perfect solution here, but I don't see any reason to heap any more responsibility or blame on to an overworked, underpaid, daily abused retail worker just trying to stay sane in one of the most soul crushing and mind-numbingly repetitive jobs I've ever known.