Here is what is unfair about your proposal...
You are, in effect, saying "I've chosen to make all my data public but I demand that only those who have made all their data public be allowed to see it."
You thereby are making a demand as to how others post their data; sorry but that is *not* your perogative.
If you realy want only those willing to share all their data too see yours the answer is very simple.
Simply make your tree private and invite only those willing to make a reciprical invitation to view their tree.
Believe me, Ancestry isn't about to implement your suggestion; the hue and cry among the membership would cause a huge loss of subscriptions and that is something they won't risk.