I was taught that you don't use anyone's work unless they give you written permission. It seemed pretty cut and dry; but once internet technology exploded, everything seemed to become a "free-for-all."
I remember getting email chain letters that I forwarded without much thought. I doubt anyone really knew where they originated. Articles, images, and other info was distributed, usually without credit being given. And everyone thought it was ok. No harm no foul, right? It became a habit not to even question it. Until someone did. People got sued and internet copyright laws were put into place. Maybe too little too late.
Today, it's impossible not to share other people's content on Facebook and other social media; after all, that is what it is is all about. But most people don't know better. They share whatever they want without a second thought.
Like all publishing platforms, Kindle Direct Publishing is adamant about using information that can be found online, even if it is your material. This is why they require you to prove the material is your own before they will publish your book. They want to keep your copyrighted material safe, and everyone else's, too.
If you want to publish your book, you need to make sure that if the content can be found online, you have a copyright statement somewhere on the website or blog that states it is your content, and that it cannot be copied without permission. This won't stop others from using your content, but at least you can prove that it is yours if you have to.