The answer to this frequently asked question is quite simple: It cannot be shared because it was not public in the beginning.
It was only published to a limited target group, in most cases to friends.
Let’s assume I spotted a truely fantastic restaurant. I want to share this interesting info with my friends, but only with my friends. I do not want to see this place too crowded. So I recommend it on Facebook but I limit the audience to “friends”.
Now if one of my friends could share this post of mine let’s say to his friends. Then he could extend my former target group to people I did not want do address. When I limit a target group for a post of mine, I do not want others to take my limitation away.
This makes absolutely sense to me, that is exactly what I intended. But the other aspect of this is reasonable as well: When a post was once published to everyone, published with no limitations, it can later be shared with any target group you like.
The important question in this whole sharing-business is: Which target group was selected in the first place? Whatever it was, it can’t get any bigger later by sharing.