"The grocery bag conundrum"

I love the U-Scan. While for some it is a frustrating mess of technological uncertainty and foibles, I find that when used and operating properly, it is a small realm of control in a world where far too many things are beyond one's control.
For instance, bagging. What I love about this new do-it-yourself approach is that I can distribute my purchases in the way that makes the most sense to me, and do away with that whole nagging question of "why did they put all THIS stuff in HERE and all THAT stuff in THERE!?!"
Specifically, why do the baggers find it necessary to put all of the heaviest items in one bag all together?...then fill one bag with nothing but a roll of paper towels and another with a bag of baby carrots? Not only is this a waste of plastic (leading to increased consumer waste and global warming -- see how these things, ultimately, make their way back into the larger picture?); but it leaves me completely unable to distribute weight evenly as I carry my packages into the house -- not to mention the obvious fact that one bag (and yes, just to further add to the very "whyness" of it all, these are rarely double-bagged) filled with canned goods, celery, a large jar of pickles and a bottle of wine is likely to bust wide open - thereby not only rendering my shopping completely futile (which necessitates re-purchase, and the cycle begins again - AARGH!), and at the very least renders it completely useless as a trash bin liner.
So, my question (as you've likely guessed) is simply, why? Why do they do that? Isn't there some kind of bagger training? And if so, why isn't the logic that so naturally springs to mind, ever applied?