I can share things with non-Gmail people too on G+. There is an option to share by email with people not using G+ yet. So it's not always-on, they just allow sharing via email.
It's important to note that those 'share by email' things are something the sharer specifically checked - it's not checked by default. If the shares are unwanted, I would complain to the person sharing them (although one imagines the email must include an opt-out link.)