It's not that you can't put it in a bag, you just need to shutdown the laptop completely before doing so.
The issue is most people don't use their laptops like that. My laptop basically only shuts down doing updates. While I can sort of understand Dells logic, it's also ignoring how people will normally use their laptops. It's really something Dell should adresse in their coming generations of laptops. No amount of writing will get users to change their behavior and even if Dell is correct, it will make their laptops seem inferior if they break because you transportet them in a bag while hibernating.
"Keep your laptop, tablet and everyday essentials protected with sleek backpacks designed to blend in to any environment."
I suspect that a lawsuit against their denial of warranty for putting a laptop into a backpack would go pretty well.