But why do they have to do this at all? If it's a shit idea, and you cannot do something without negative side-effects of it, can't you just not do it? Why these companies absolutely have to do this?

Well, they want to use the book they own in digital format. They own it, they get to decide what to do with it.

What's shit about it ? They're buying books that would be headed for the pump or trash heap anyway. Millions of books are trashed or pulped every day.