Because each additional story shared add contents to your friends feed ? And some things you don't want to share are the one that would interest your friends the most.
("friends" in the Facebook sense, of course).
And you don't have to click to share. You just have to do something in one apps linked somehow to your facebook account. Or leave a comment on a third party site. Or..
Yes, you can be attentive to all these, but that's not what I would call easy for everyone.
It would be intentional because more shares = more traffic = more money for Facebook. Longer term, it means less expectation of privacy and even more opportunities to publish even more stuff to more people. Zuckerberg has pretty much explicitly stated that this is the goal.
And those explicit confirmations contain so much noise that it's often quite difficult to tell what you're confirming.
Further, I don't even understand the accusation. You have to click "Share" on a post to share it, and even then, there's an additional, quite explicit confirmation required.