I very much understand and respect the reasons people keep their trees private. The private tree owners who take and do not share are RARE, and all private tree owners should not be judged by those few.
My last word to those complaining about private tree owners - Research is not copying other peoples trees. Sharing does not mean I show you mine, you show me yours, we click and copy, no social interaction necessary. Instead of obsessing about what might be hidden in a private tree, why not focus on what is being made public. When you find a photo, document, or story involving your ancestor, do you contact the person who originally uploaded it to thank them and explain how you are related? You may be surprised at what you will learn, as folks who have been researching their families for years tend to have a lot of info that is not attached to their public tree. Many of the stories attached to my public tree are my transcriptions of original documents that have been very difficult and/or expensive to access. I do not make images of those documents publicly available, but am very willing to personally share copies with other descendants. Those story transcriptions have been copied many times over by others, yet not one person has ever contacted me to ask about the original documents.