I don't understand if this is a feature you want added which will apply to everyone (ie.. no private trees will be able to view or copy from any public trees) or whether it is a feature you want put in place just for yourself.
Either way it cannot work.
It doesn't matter how much YOU hide your tree from private tree people, because once you share your tree with ANYONE (public or private) they then have the right to share with whomever they want.
Say you have a public tree and I cannot view it or copy from it, but your new found cousin has a public tree and you share info/photo's from him/her. What is to stop me contacting your cousin and swapping some info with them and accessing your info/photo's anyway.
You honestly cannot expect every single person you share/swap information with to promise never ever to share/swap that information with anyone else..