The OP really annoyed me! I honestly can't even fathom in the remotest reaches of my mind how someone could in any justify not sharing because of such a selfish reason! I once contacted a match and they didn't want to share because they were adopted and their birth mother wouldn't allow them to share anything that might connect that person to her family. IMO that was sick and disgusting of the mother but I could at least understand that.
For me, I enjoy genealogy because it's a puzzle and the OP acts like someone just copied her answers to the New York Times crossword. If someone wants copy fine. There are people who just want to fill out their trees, so let them. They don't know where any of that information came from. But to act like that information is private property is unpardonable. Unless you are doing research you plan to publish in a book I don't understand why anyone would refuse to share, when all those non-sharers out there are ALL taking advantage of those who do share.
Ancestry is partially to blame, because you can't merge two trees on ancestry so there is no point to make a 2nd tree of iffy info to add to the main tree later. Probably because ancestry makes it's money off of all of our trees. I honestly wish that Ancestry would not allow people to link locked trees to their DNA.