I don't think that's how sharing someone else's post works. If I make a friends-only post, and you share it to your friends, only the intersection of our friends can see it. I agree this is maybe not as clear as it could be on the UI, but I believe that's how it should work. If you find otherwise, would you mind filing a bug at www.facebook.com/whitehat?
https://fbcdn-sphotos-a.akamaihd.net/hphotos-ak-snc7/374152_...