Instead of disallowing it outright, I'd rather they make it clear that you're sharing content with a wider audience. One solution would be to require that the owner approves the share, similar to how tagging is handled.
This post was originally shared with a limited audience –
remember to be thoughtful about who you share it with.
This, in my opinion, is where Google+ is failing: sharing makes it very easy to make private posts visible to an audience larger than the author intended.
In addition to G+ reminding users when resharing a limited post that it was intended for a smaller audience and to be mindful of that, the original poster can also disable resharing entirely.